MBB vs Big 4: what's actually different

Before the detail: I talked this whole question through with Heinrich from Firm Learning, who spent his career at McKinsey. We answered the same set of questions from opposite sides — his from MBB, mine from Deloitte. It's worth twelve minutes if you're actually choosing between the two.

▶ 12:05

1. MBB firms are far smaller — which is why they're more selective

This is the fact that explains most of the others. The top three strategy firms are roughly a tenth of the size of the Big 4.

As of 2018, McKinsey had around 27,000 employees, BCG around 20,000 and Bain around 8,000. Deloitte, PwC, KPMG and EY each employed somewhere between 200,000 and 300,000 people.

The revenue gap tells the same story from the other end. None of the three publish figures, but the working estimates put McKinsey at roughly $10bn a year, BCG around $8bn and Bain around $5bn. Set that against the Big 4, whose consulting arms alone are multiples of it. Small firm, very large fees per head. ▶3:02

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The consequence is arithmetic. The Big 4 must recruit a very large number of people every year just to sustain their size. MBB need far fewer. So purely as a numbers game, MBB are more selective — not necessarily because the people are categorically different, but because there are fewer seats.

Worth knowing, though: the Big 4's consulting divisions are bigger than MBB — by headcount and by revenue. For Deloitte and PwC in the UK, consulting is among the largest revenue drivers, bigger than audit. The "Big 4 are just accountants" line is a decade out of date, and some of their own clients still believe it.

Size also shows up in what each firm can sell you. The Big 4 — Deloitte, KPMG, EY and PwC — account for more than 40% of worldwide consulting revenue between them, and over the last decade their consulting arms have bought their way into supply chain, innovation, proposition design, brand and marketing. That breadth is the thing MBB can't match. What MBB can do is charge more, which is why the salary gap opens up as you get senior. ▶1:53

Does your university actually matter?

This is the question I get most, and the honest answer is: less than you think, and less than it used to. I studied at Imperial, but the people who joined alongside me came from all over. The Big 4 did once recruit almost entirely from Oxbridge, the Ivies and Imperial — now they deliberately hire much more widely, because different backgrounds bring different viewpoints to a client, and clients notice.

A good university helps you get screened in. It isn't the whole picture — they're also looking at experience, extracurriculars, achievements and skills. One thing worth knowing is that this varies by country: in the US the differences between universities are far more salient than they are in, say, Germany, so target schools carry more weight there than they do in Europe. If you want proof rather than reassurance, go on LinkedIn and look at where people at MBB and the Big 4 actually studied. ▶5:10 I got in with no business degree and no consulting internship, so I'd rather you didn't rule yourself out on this.

2. Both do strategy and implementation — they're just known for different halves

This is the bit the internet gets wrong, and it's the reason people talk themselves out of the right job.

MBB do strategy and implementation. The Big 4 do strategy and implementation. What differs is reputation and centre of gravity, not the menu.

It doesn't help that "strategy consulting firm" is a slightly misleading label to begin with. Heinrich's point, from the inside at McKinsey, is that the more accurate description would be top management consulting — the vast majority of the business is working with the top management of large clients on whatever is at the top of mind for those senior leaders. Sometimes that is a strategy question. Often it isn't. ▶2:35

MBB are known for strategy because most of their work sits on the biggest questions a company has: strategy, organisation, marketing, operations, digital, M&A. C-suite problems. Small teams, senior audiences, sometimes board members in the room. But every MBB firm now runs implementation arms too — and if you join one, you may well find yourself on a project that's about execution and bringing an idea to life, often an idea a previous team sold in. That is not the brochure, but it is the reality. ▶8:23

The Big 4 are known for implementation because that's where the volume is: rolling out a process or a technology, restructuring business units, running the transformation. Which means Big 4 consultants more often work with the client's middle management than its C-suite. But the Big 4 absolutely do strategy — Strategy& at PwC, Monitor at Deloitte. Smaller share of revenue, often bought in rather than grown, and the people there work across sectors much like MBB.

So "MBB is strategy, Big 4 is delivery" is a reputation, not a rule. Pick on it and you'll be wrong about your own day-to-day.

The better question: do you want to be a consultant, or an operator?

Here's the split that actually predicts whether you'll be happy.

A consultant is at their best in the thinking: framing the problem, finding the answer, making the argument, handing it over. The satisfaction is in the quality of the recommendation. You want to be in the room where the decision gets made, and you're content that someone else carries it out.

An operator only really feels finished when the thing works. You want to own the outcome, live with the trade-offs, be there when it hits reality and something breaks. A perfect deck that nobody implemented feels like a loss.

If you're a consultant at heart, the shorter, more strategic MBB cycle will suit you, and the Big 4's strategy arms will too. If you're an operator, the longer Big 4 engagements are the better training ground — you stay long enough to see whether you were right, which is the only feedback loop that actually teaches you anything.

I found this out the slow way. I nearly turned down tech consulting because I'd heard it was "just implementation" — that was wrong, and it would have cost me the way in. It also turned out to be the part I was built for: I went from consultant to operator, and the implementation work is precisely what made that possible.

Team size is where the difference gets concrete. On a strategy or prime project at the Big 4 you're in a small team of two to four — a partner holding the client relationship, a manager, and an analyst or consultant. That's close to how MBB run almost everything — a typical MBB team is two to four consultants including the project lead, with one or two partners dropping in once or twice a week. Being one of three people means real responsibility from your first project, and an audience of senior executives, sometimes board members, from very early on. But on operations and technology work, a Big 4 programme can run to 300 people, split into work streams, with your immediate team somewhere between four and twenty. You are far more likely to be managing, and being managed, inside a structure. ▶10:32

3. Generalist versus specialist — the difference that shapes your career

This is the one that actually changes what your life looks like, and almost nobody weighs it properly.

Big 4 consultants often spend 6–12+ months on the same client, embedded almost like an employee. You specialise in an industry early. The work can get repetitive — but you develop real depth, and you're still there to see whether what you recommended actually worked.

MBB consultants typically spend 2–4 months on a client, sometimes less, and rarely specialise until much later. The work is more varied. You also frequently leave before your recommendation meets reality.

Neither is better. They suit different people. If you get bored quickly and want breadth, the shorter cycle is a gift. If you want to own an outcome and become genuinely expert in something, the longer engagement is.

4. Up-or-out — and why the UK is different

Up-or-out is real at MBB: at every level, some people will be asked to leave. It's less brutal than it sounds, because the feedback is constant and you generally know where you stand, and the firm will help you land somewhere else. But it is the rule of the game.

What softens it is what happens next. MBB firms have a very large alumni network and HR will work with you to find the next job — being managed out of McKinsey is not the same event as being managed out of most companies, and everyone in the market knows it. ▶6:07

The Big 4 has a version of this in a lot of countries. In the UK it works differently, and almost nobody tells you this before you join: it is genuinely difficult to be let go for performance here. The Acas code of practice means a firm has to show you've consistently underperformed, give you chances, and put you on a support plan first.

What you get instead is structure. Leadership sit down once a quarter to talk through how everyone is doing and who needs support. Promotion runs once a year at the junior grades, early promotion included, against a defined set of guidelines for each grade. It is slower and safer — and if you want to move up, it means you know exactly what you have to evidence. ▶7:29 That clarity is the thing I'd use if I were you: the jump from consultant to manager is where it matters most.

So which should you go for?

Go for the one that matches how you want to work, not the one with the better logo in a group chat. And if you're choosing between an offer in hand and waiting a year for a "better" firm — take the offer. A year is an expensive way to buy prestige, and you can always move.

I got into the Big 4 with no business degree and no consulting internship, in tech consulting I initially didn't even want. It worked out.