"Big" is a relative term. I wear a 34F and don't look disproportionate. It may however be a difficult size to buy a bra for, I recommend checking out a local maternity store. The people there will be understanding and helpful (or at least they should be) if you explain that you're not pregnant you just need a bra. The last time I went to buy one the girl working there said they get a lot of people with the same problem and that 34F is actually a pretty common size.
We're all made differently, don't sweat it!
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