The thing with smaller or even mid-sized firms is typically you are not only performing one accounting function (audit, tax, bookkeeping, etc).
Working in smaller/mid-sized public accounting firm, you would normally have the opportunity to provide all types of service. Typically in smaller firms you have a more personable relationship with clients and get to play an integral part in onboarding new clients and seeing the whole process through.
From my understanding of Big 4, everything is usually nice and neat and already laid out for you. In other words, you don't have to do much digging or dirty work yourself at Big 4. A lot of analyzing documents and spreadsheets, but the clients pretty much do what you ask them to do or most of the time already have everything prepared for you to audit.
At the firms I've been with, I've onboarded clients. Prepared all types of tax returns from 1065, 1120, 990, 1040, and 1041, etc. I've also performed a variety of assurance services from start to finish including audits, reviews, comps, and even AUPs and forensic engagements. I've created an entire accounting system for a $10M+ company using job costing and established a payroll system for 60+ employees. (This is with about 5-6 years experience).
I don't think Big 4 gets to do things like that and have a more narrow skill set. Unless of course, you get into the actual big firm stuff like M&A or consulting, etc.
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