I am not from the US. But I have some experience at a Big 4 in Germany.
Tax work at a Big 4 is very specialized. You work in a “service line” which specializes either in a certain type of client (real estate funds, private equity funds, domestic corporations, international corporations, executives) or in a certain tax problem (turnover tax which is very important in Europe, transfer pricing, mergers and acquisitions). They also divide the work further within a service line e. g. one associate works only on mid-sized corporations, another only on big corporations, or they want you only for preparing legal opinions on tax problems or reviewing contracts for due diligence.Your work experience is therefore very focused on one “function”. You may well only get to see one type of tax return in your whole carrier there.
I think this makes it hard to change from a Big 4 to a smaller firm because smaller firms know that you are lacking the kind of diverse tax experience they need. Also, “small company problems” are very different from the kind of problems that a Big4 firm deals with. However, it is much easier to change from a Big 4 firm to big industry because big industry has a very similar internal structure and they are looking for those very specialized people not for the allrounders from small firms. And big industry is where the big money is, right?
I also found that they review your work a lot more at a Big4 than at a smaller firm. At the Big4 I spent approximately 40% of my work time making annotations for my tax manager so they could review my work faster. Everyone was required to do that. I mean, c'mon! At a small firm, I would do almost twice as much work in the same time.
They have also a different hierarchy at the Big4. Small firms lack the “tax manager level”. As an associate I got my own business cards and was allowed to deal with my own clients directly after three months on the job. I had to organize my work by myself from the beginning. A partner reviews my work and I report to him directly. At the Big4 they told me that I would have to stay for at least one or two years before I get to work on anything remotely important. No client contacts, no responsibility etc. However, they offered a 25% higher salary than the small firm.
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