Choosing the wrong accountancy firm comes at a higher price than just the cost of hiring. Audit fees increased by 27% in the UK over one year. Mid-tier firms accounted for 18% of public company audits and 26% of foreign private issuer audits in the U.S. for 2024, up 14 percentage points from the previous year. 

Businesses are making the switch, and for good reason. It’s not a matter of size or prestige. It’s all about the revenue level, compliance needs, and growth trajectory. 

This guide helps you get cost benchmarks, partner access information, client segmentation by revenue, and a decision matrix, all you need to make the right call. 

Big 4 vs Mid-Tier Accounting Firms at a Glance

Factor	Big 4 Firms	Mid-Tire Firms
Typical Client Revenue	$250M+	$5M–$250M
Audit Complexity	Very High	Moderate–High
International Presence	Extensive (142–152 countries)	Selective (100–160 countries)
Partner Accessibility	Lower (~10:1 staff ratio)	Higher (~4–5:1 staff ratio)
Cost	Premium	Moderate
Implementation Speed	Slower	Faster
Best For	Public companies, multinationals	Growth businesses, SMEs, PE-backed firms
Fortune 500 Audit Share	100%	0%
2024 Revenue Growth	5.7%	6–9%

The selection between Big 4 accounting firms (Deloitte, PwC, EY, KPMG) and mid-tier firms (BDO, Grant Thornton, RSM, Mazars) will depend on the size of business, budget, regulatory requirements, and complexity. 

Big 4 firms: Big 4 firms are best suited for publicly listed, multinational, and large companies that require global experience, institutional credibility, and complex financial services.

Mid-tier firms: Ideal for SMEs, startups, and expanding businesses seeking affordable, adaptable, and customized accounting services, mid-tier firms are the best choice.

Outsourced Accounting Firms: The third option is outsourced accounting solutions (such as those provided by Aone Outsourcing Solutions), which provides finance function expertise without the overhead of either type of company. 

What Are the Big 4 and Mid-Tier Firms?

The Big 4

The four largest professional services networks in the world are known as the Big 4 and include Deloitte, PwC, EY, and KPMG. Together, they generated revenue of more than $212 billion in 2024, with more than 1.4 million professionals working in 142–152 countries and auditing each and every Fortune 500 company.

They are designed to be scalable, complex, and institutionally credible. The Big 4 are here for you when your business is in 10+ countries, publicly listed, or is about to go public.

Firm	2024 Revenue	Employees	Countries
Deloitte	$67.2 billion	~460,000	150+
PwC	$55.4 billion	~328,000	152
EY	$51.2 billion	~395,000	150+
KPMG	$38.4 billion	~275,000	142

Sources: Statista – Big 4 Revenue, Deloitte Global Report, PwC Annual Review

Mid-Tier Firms

Mid-tier firms, also known as challenger firms or second-tier firms, fall between the Big 4 and local boutique practices. They are far from small: BDO brings in $14 billion annually and has an office in 160+ countries.

These companies employ the same International Standards on Auditing (ISAs) as the Big 4 firms, are subject to the same regulation by the FRC and the PCAOB, and are recruiting more and more Big 4-trained talent. The only real difference is who they serve and how they serve them.

That said, the revenue of mid-tier firms is increasing by 6-9% per year, which exceeded the 5.7% growth of the Big 4s, as businesses actively opt for them over the Big 4s. 

Firm	2024 Revenue	Employees	Countries
BDO	~$14 billion	115,000+	160+
RSM	$10 billion (FY2024)	41,420+	120+
Grant Thornton	$8 billion (FY2024)	72,858	145+
Mazars / Forvis Mazars	~$5 billion	50,000+	100+
Crowe	~$5 billion	40,000+	130+

Sources: BDO Global, RSM Global, Grant Thornton International

The achievement gap is measurable, but it is closing. The growth of mid-tier firms was faster, despite the Big 4’s combined growth of 5.7%, compared to 6-9% fee income growth across the four firms. Private equity has noticed, too: 25% of the mid-tier firms are now privately financed, and PE funds have aided fast-track consolidation.

Side-by-Side Comparison Table

Feature	Big 4 Firms	Mid-Tire Firms
Examples	Deloitte, PwC, EY, KPMG	BDO, Grant Thornton, RSM, Mazars, Crowe
2024 Global Revenue	$212B+ combined	$5B–$14B per firm
Global Presence	142–152 countries	100–160+ countries
Employees (Global)	275,000–460,000 per firm	40,000–115,000 per firm
Best For	Large enterprises, multinationals, and listed companies	SMEs, startups, mid-market, growing businesses
Cost	Premium — highest in the market	Moderate, competitive value pricing
Staff-to-Partner Ratio	~10:1	~4–5:1
Partner Access	Limited — mostly junior teams	Direct — partner-led delivery
Service Flexibility	Lower — standardized methodology	Higher — tailored to client needs
Fortune 500 Audit Share	100%	0%
SEC Market Share (2024)	~69.5% of audit fees	~18% of the U.S. market
Market Fee Growth	5.7% (stagnating)	7.2–8.8% (accelerating)

7 Critical Differences Between Big 4 and Mid-Tier Accounting Firms

1. Pricing Models and Engagement Costs

Cost is where the gap between firm types is most visible and most consequential for growing businesses. 

The Big 4 firms command higher prices due to their worldwide infrastructure, investment in the brand, and their multiple review systems. The average audit fee that PwC billed to its SEC-registered clients in 2024 was $5.52 million. Total FTSE 350 audit fees reached £1.4 billion in 2023, up 27% in a single year, with 98% going to the Big 4.

Mid-tier companies provide the same technical quality at a much more affordable price. The average audit fee for businesses generating between $25M and $200M in revenue is between $60,000 and $200,000 per year. The difference between the fee for a company with a turnover of less than £30M and a Big 4 firm can be significant for companies in the UK.

The business implication: If the engagement doesn’t need to rely on Big 4 infrastructure, global coordination across 50+ countries, Fortune 500-level complexity, or institutional investor mandates, then you’re paying for overhead that delivers no value. 

2. Partner Involvement and Staff Leverage

This is one of the most overlooked differences between firm types.  

Metric	Big 4	Mid-Tier
Staff-to-Partner Ratio	~10:1	~4–5:1
Day-to-Day Contact	Manager / Senior Associate	Partner-led
Partner at Fieldwork	Rare	Common
Long-term Team Continuity	Low (high staff turnover)	High

At a mid-tier firm, the partner who pitches is the partner who delivers. Your account is handled by the same senior team every year, and they have developed institutional knowledge about your business that you can’t get through a handover document. 

3. Global Compliance Capabilities

Both types of firms follow the same International Standards on Auditing and are subject to the same regulatory scrutiny by the FRC (UK) and the PCAOB (US). However, their ‘real world’ skills vary by scale.

Multi-country compliance is deeply integrated in the network of Big 4 firms, with a strong capability to coordinate IFRS reporting, SOX compliance, SEC filings and transfer pricing across 50+ countries at the same time. There are several businesses that have true multi-country complexity, and this coordination is hard to duplicate elsewhere.

Mid-tier firms do a relatively good job of international compliance for businesses doing business across 1-5 countries. There are companies with extensive international networks, such as BDO (160+ countries), RSM (120+) and Grant Thornton (145+). It is the most complex multi-jurisdictional engagement among dozens of legal entities and multiple regulatory regimes, in which they face resource constraints. 

4. Industry Specialization

Mid-tier organizations have developed significant vertical expertise in areas that are most relevant and important to the growth of their businesses:

Technology and SaaS — revenue recognition, R&D tax credits, equity structures Manufacturing – inventory accounting, cost management, cross border supply chains

Healthcare— that is, regulatory compliance, grant accounting, and NHS-related entities

Real estate – property valuation, REIT structures, real estate accounting for development

Private equity portfolio companies – Financial reporting, covenants compliance, and exit preparation

Big 4 firms have a greater degree of specialization across a wider range of sectors, such as financial services, insurance, and listed infrastructure, where regulatory requirements may call for their involvement. 

5. Client Response Times and Accessibility

Accessibility — the ability to get a response from a senior level when it matters most, is a key difference that consistently distinguishes mid-tier companies from growing enterprises, according to reports.

Big 4 client service is typically available through ticketing channels, account managers, and formal escalation channels. A senior partner will need to be booked for the phone. The response time of complex queries can take days.

Middle-tier firms use direct lines. The mobile number for your engagement partner is on the pitch document. These questions are answered on the same day. This gap has become apparent over the years — particularly when it comes to raising funds, making acquisitions, and providing the breathing room necessary to meet rapid compliance deadlines.

6. Technology and Automation Stack

There has been a strong overall emphasis on audit technology, artificial intelligence-driven analytics, and cloud reporting systems for both types of firms.

Each of the Big 4 has a proprietary platform designed for large and complicated audits, such as Deloitte’s Illuminate, PwC’s Halo, and KPMG’s Clara. These are effective weapons used in enterprise-level engagements.

Mid-tier companies are increasingly embracing best-in-class third-party platforms—those that the finance industry uses today to operate in the cloud—to make integration with rapidly expanding businesses easier. At top-tier companies, ERP connectivity, automated transaction testing, and real-time dashboards are now the norm, but without the enterprise overhead. 

7. Scalability During Business Growth 

Growth Event	Big 4 Advantage	Mid-Tier Advantage
IPO Preparation	Strong: investor credibility	Limited brand recognition
Acquisition (domestic)	Adequate	Strong: faster, more flexible
International Expansion	Strong: global network	Adequate for 1–5 countries
PE Investment Round	Adequate	Strong: direct senior access
Rapid headcount growth	Slower to adapt	Faster, proportionate response

Big 4 vs Mid-Tier Accounting Firm Costs: What to Expect 

Service	Big 4 Firms	Mid-Tier Firms
Annual Audit (mid-market)	$500K–$5M+	$60K–$200K
Tax Advisory (annual)	$200K–$1M+	$30K–$150K
CFO / Finance Advisory	$300K–$800K	$50K–$200K
Outsourced Accounting	Not typically offered	$2K–$10K/month

What Drives the Fee Difference?

Big 4 fees are more than just technical work. You are paying for:

• Global infrastructure across 142–152 countries
• The brand premium of institutions’ needs.
• Multiple levels of internal checks
• Adhering to the highest level of regulatory standards

If these are things your business actually requires, the payoff is worth it. In businesses that don’t, it is overhead with no return.

When Is the Premium Justified?

Big 4 premium pricing is acceptable when your auditor’s brand affects a financial result, in the context of IPO pricing, institutional credit facilities, requirements for listed companies or regulatory requirements. In such cases, cost is not an option; it’s a business requirement.

In all other cases, mid-level firms provide the same level of technical expertise at a much reduced price. 

Which Should You Choose?

Choose Big 4 Firms If:

• You’re a publicly listed company or IPO planner — auditor brand matters a lot to investors — and virtually every company on the S&P 500 is audited by the Big 4.
• You run your business in several countries and need to coordinate the tax, audit, and compliance activities globally on a large scale. You have a cross-border business and need to coordinate and run tax, audit, and compliance activities across all countries, at scale.
• You’re looking for very complex, specialized services — transfer pricing for multi-national setups, financial instrument valuation, complex forensic work, or cross-border M&A.
• Your industry is regulated — Banking, Insurance, Listed infrastructure — Regulators/exchanges have preferences regarding auditor type.
• Big 4 Sign-off is required for Institutional Investors, Credit Facilities, or Large Contracts. 

Choose Mid-Tier Firms If:

• You are a small- to medium-sized business, startup, or private company and have to work within budget and quality constraints rather than brand recognition.
• You want access to your senior partners throughout your engagement, not just at billing time.
• You’re expanding your business or looking for investment — you don’t need enterprise-level pricing for mid-tier businesses.
• You prioritize continuity — a team that truly knows your business through the years.
• Your accounting requirements are complicated, but not worldly complicated. 

Consider Outsourced Accounting If:

• CFO-level expertise at a part-time cost — an outsourced finance function provides strategic financial leadership when it’s needed
• You are a start-up or scale-up that is dealing with cash flow, reporting, and investor relations without having a large internal team.
• You would like to free up internal resources to outsource bookkeeping, payroll, management accounts, and compliance.
• You require scalability — outsourced teams are scalable without the need for recruitment or redundancy costs.

If you’re considering an outsourced or co-sourced finance function, talk to Aone Outsourcing Solutions about whether an outsourced or co-sourced finance function is right for your business:

Revenue-Based Decision Framework

Revenue Range	Recommended Approach
Under $2M (startup phase)	Outsourced accounting + boutique or regional firm for compliance
$2M–$25M (growth stage)	Outsourced finance function + mid-tier firm for audit/advisory
$25M–$250M (mid-market)	Mid-tier audit firm + outsourced CFO or specialist support
$250M+ or IPO-bound	Big 4 audit + transition planning for institutional requirements

Why Businesses Are Shifting to Mid-Tier Firms

The cost of Big 4 fees is too high for most businesses and will not be sustainable. 

1. Big 4 Fee Inflation Is Unsustainable for Most Businesses

Total FTSE 350 audit fees rose 27% to £1.4 billion in 2023. The FRC noted that fees were rising and that clients were growing concerned about this. However, in the U.S. market, mid-tier firms saw a 14-point increase in their share of foreign private issuer issuances, rising to 26%, due to real client interest in diversifying. 

2. Mid-Tier Capability Has Dramatically Improved

Starting around 2020, mid-tier companies began closing the capability gap by recruiting Big 4 graduates, investing in technology that met PCAOB requirements, and establishing teams of specialists. The outcome: mid-tier companies achieved 25% profit growth in 2024, compared to the Big 4’s 5.7%, and posted revenue growth of 6-9% per year, compared to the Big 4’s 5.7%. 

Source: Distinct Recruitment – 2025 Top Firms Survey

3. Private Equity Is Accelerating the Transformation

Private Equity is driving transformation faster.

• PE has gone into the middle-market accounting business:
• In March of 2024, Grant Thornton US sold a majority stake to New Mountain Capital (NMC).
• In late 2024, Grant Thornton UK decided to sell a majority stake to Cinven for approximately £1.3 billion.
• Cooper Parry raised investment funding from Carlyle Group (2024)
• In the 2025 Top Firms survey, 17 PE-backed firms collectively reported a 19.8% increase in fee income.

Source: Accounting Today – PE in Accounting | Financial Times – Grant Thornton Cinven

4. Regulatory Pressure Is Leveling the Playing Field

The PCAOB and FRC have been tough on large firms, requiring them to rethink their business operations and talent. As regulators probe the Big 4 firms over the quality of their work, this is driving stronger sentiment for companies to hire non-Big 4 firms as a viable, less risky alternative to the Big 4, rather than a compromise. 

Reference: FRC Audit Firm Monitoring | PCAOB Inspections

When Outsourced Accounting Is Better Than Either Option

There’s a third option that’s becoming more popular for businesses—outsourcing their accounting and finance roles —and it’s gaining mainstream traction.

Outsourced accounting involves delegating the entire accounting process (from bookkeeping and management accounts to payroll, VAT returns, and CFO coordination) to an accounting specialist or service provider instead of creating an in-house accounting team or paying retainers to a firm that is not independent of the client.

When Outsourced Accounting Makes More Sense?

• Your company revenue is under $25M, and you are not yet ready for a statutory audit team on retainer.
• Need a financial strategy at the level of a CFO but don’t need to hire a CFO to come on board full-time.
• You wish to increase or decrease your finance function without recruitment or redundancy expenses.
• Your in-house team isn’t scaling, and a limited budget is taking a toll on management time. Your internal team is not scaling, and your finances are robbing management of their time. 

What Outsourced Accounting Covers

Service	Details
Bookkeeping	Real-time transaction processing and reconciliation
Management Accounts	Monthly P&L, balance sheet, and cash flow reporting
Payroll	Compliant processing for teams of any size
VAT and Tax Compliance	Accurate filing and strategic tax planning
Outsourced CFO	Fractional or full-time strategic financial leadership
Audit Preparation	Records organized and audit-ready before your auditor arrives

Explore Aone’s full range of outsourced finance services: https://www.aoneoutsourcing.com/

Further reading: ICAEW – Finance Outsourcing Guide | ACCA – Outsourcing Finance Functions

Which Type of Firm Is Right for Your Business?

Businesses That Typically Choose Big 4 Firms…

Characteristic	Typical Profile
Annual Revenue	$250M+
Countries of Operation	5+
Public Reporting Required	Yes
Regulatory Complexity	High
Investor Type	Institutional
Audit Mandate	Often contractually required

Businesses That Typically Choose Mid-Tier Firms…

Characteristic	Typical Profile
Annual Revenue	$5M–$250M
Countries of Operation	1–5
Public Reporting Required	No
Growth Stage	Expansion / Scale-up
Investor Type	PE, VC, or private
Priority	Cost efficiency + senior access

Which Accounting Firm Should You Choose? A Decision Matrix

Your Business Situation	Recommended Option
Preparing for IPO	Big 4
Public company statutory audit	Big 4
Multi-country tax planning (10+ jurisdictions)	Big 4
International M&A	Big 4
Fast-growing SaaS company ($10M–$100M)	Mid-Tier
Mid-market manufacturer	Mid-Tier
Private equity portfolio company	Mid-Tier
Family-owned business	Mid-Tier
E-commerce or service business	Mid-Tier
Early-stage startup needing finance infrastructure	Outsourced Accounting
SME needing bookkeeping + management accounts	Outsourced Accounting
Business preparing for first audit	Outsourced Accounting
Owner-managed company needing compliance	Outsourced Accounting

Pros and Cons Summary

Big 4 Firms

Big 4 Advantages:

• A broad expertise in global regulation in 142-152 countries.
• Unparalleled credibility for IPO, listed company, and institutional investor needs
• Expert knowledge of transfer pricing, IFRS, ESG reporting, and forensic accounting.
• A wide range of industry benchmarking information from across the industry.
• Regular FRC & PCAOB inspections establish consistent-quality floors. 

Big 4 Limitations:

• The “premium” price is one that most businesses with under $250M in revenue cannot afford.
• Direct partner access is lower on most engagements, with higher staff leverage.
• With standardized service delivery comes reduced flexibility for quick-moving businesses.
• Over time, as staff turnover increases, institutional knowledge is passed down to other staff members.
• Smaller clients may find that their efforts are not rewarded as much as those of larger clients.

Mid-Tier Firms

Mid-Tier Advantages:

• Much cheaper than Big 4, comparable technical work.
• Partner-led delivery with 4-5:1 staff ratio – a more senior attention throughout
• Follows the same ISA requirements and regulations as the Big 4
• Accelerated and agile response for business changeovers
• Professional services showed the strongest growth in annual revenue, 6-9%. The fastest-growing were professional services, which saw an annual revenue increase of 6-9%. 

Mid-Tier Limitations:

• Local market depth in some countries may not match Big 4 presence
• Worsening of brand awareness issues regarding IPO and institutional fundraisings
• Limited opportunities for highly complex multi-jurisdictional engagements
• Support for advisory services outside the core has narrowed and is less than that of the Big 4. 

Outsourced Accounting Services 

Outsourced Accounting Advantages:

• 20–30% cost saving when compared to an equivalent number of in-house employees
• Takes advantage of instant scalability without recruitment, training, or redundancy costs.
• World-class CFO level capability without a full-time commitment to salary.
• The most modern cloud platforms with real-time financial reporting.
• Redundancy inside the built-in system, no single point of failure.
• Opens opportunities to work with both Big 4 and mid-tier audit teams

Outsourced Accounting Limitations:

• Is not an alternative to a statutory audit; auditor independence rules require a different firm
• Needs to be well supported when they are first set up and they need to know how to do things
• Best for businesses that have moderate, not high, transaction volumes. 

Frequently Asked Questions

Q1: Can mid-tier handle international compliance? 

Yes, in most business situations. The mid-tier firms use the same International Standards on Auditing (ISAs) as the Big 4, are subject to the same oversight, and often have employees from Big 4 firms. Internationally recognized and active in 120–160 countries, RSM, BDO, and Grant Thornton are all part of the group. 

Q2: Is a Big 4 firm worth the cost? 

Big 4 firms will charge the most of any firm in the market. In 2024, PwC’s audit fee per SEC client averaged $5.52 million. Regional mid-tier firms generally audit private mid-tier firms of the same size for $60,000–$200,000 per year. Many clients don’t benefit from the overheads involved in global infrastructure, brand investment, and multi-layered review processes that are part of the Big 4 fees. 

Q3: What are the alternatives to Big 4 accounting firms?

Mid-tier firms like BDO, Grant Thornton, RSM, Mazars, Moore, Baker Tilly, Crowe, Nexia, regional boutique firms, and outsourced finance function providers are the main options. Mid-tier companies offer the widest range of global reach, services, and pricing options. Add accounting function expertise through outsourcing, including bookkeeping, management accounts, payroll, tax compliance, and CFO solutions, with Aone Outsourcing Solutions. 

Q4: What revenue size typically justifies hiring a Big 4 firm?

For practical purposes, Big 4 engagement economics are relevant to businesses with revenues over $250m, or to any business irrespective of size, if it’s publicly listed or planning IPO, or is mandated to have a Big 4 auditor as part of its contractual terms. Any firm below that threshold is usually better suited for commercial purposes. 

Q5: Which accounting firm type is better for private equity-backed companies?

From an investment perspective, it is far better to partner with mid-tier companies for PE portfolio companies. PE sponsors are keen to have direct access to partners, which mid-tier firms structurally do better; they are also interested in faster communication, cost savings, and targeted financial reporting. Market data show that 26% of foreign private issuer audits were conducted by mid-tier companies, representing a 14-point increase over the past year.