Fact that "Nothing can be learned in small firms" is completely wrong.
In small firms, you have all types of work like Accounts maintanence for clients, Return filing works (TDS,
Service Tax), Registration matters (ST& VAT registration), Income Tax Returns (Salary, HP, Business,
Capital Gains), Companies & Trusts audit, Bank Audits, Stock audits, Certification Issues (Form 15CB)
Statutory audit of Companies, Tax Audits Etc., With this kind of exposure, you can survive in any
Industry (Banking, Manufacturing, Trading etc.,) . Also it will help you to prosper in practice.
Only disadvantage is that there is no RECOGNITION in industry and interviewer will definitely
come to know about your potential after interview. In Big 4, you can learn things relating to a particular
area like (Statutory audit, Taxation) whereas you can become all rounder if you do your articleship
in Small / medium size firms..
It is the person's willingness to learn only matters and not the place he is getting trained.
In my opinion, no matter where you do your articleship, it is the determination and willingness to learn are in built in a new entrant to the profession, even a person doing articleship from remote areas can do wonders in CA Exams. This is evident many a times from the past results of the CA Exam.
It is not that only Big firm's experience is good and rest all are waste...
You can see many people from Big firms fail and also people from small/medium firms have come out with flying colours.
Only the result matters....
All the Best.