Actually girls wear bigger bra sizes so that they can stuff them. They don't normally wear a smaller size because (A) It's VERY uncomfortable for even a normal size bra, how do you think a bra that's too small will feel? and (B) It's WAYYY too noticeable that you're trying to make them look bigger, and (C) You'll look stupid with have your breast bulging out of your chest, trust me, i know girls who do it