4'11", 125 lbs, 34B. Is 400 cc too big for my size? (Photo)

Bra manufacturers do not use CC sizing so finding a 1:1 correlation between the cup size and CCs is impossible.   At your next pre-op visit, try on a profile bra with various 400 cc sizers under a tailored blouse, tight sweater, swimsuit, workout clothes, and other garments you typically wear so that you have the best idea of whether the garments fit as desired with the implant volume selected. Choosing an implant based on an aesthetic goal that exceeds your anatomic capacity may lead to complications and possibly revision. 

always tough to say by photos, but if that picture is your dream , then go for it!!!  good luck.....