Big 4 CDD is normally focused on mid-market transactions, helping Corp Finance write an IM or helping Accountants review the forecasts. When the going gets tough, the big guys (TPG, Apax, etc) call the A team - Bain, McK, etc. Bain, McK would not like to think of themselves as competing with pwc, and are unlikely to hire from there since they would not be recruiting for CDD specialists anyway. McK normally want polyglots who can move into any service line in any industry in any location. a CDD training at big 4 doesn't give you that. The size of the dept vary from 20-120. People can stay ther quite a while 'cos it's not so up or out as strat houses.