Boobs are boobs - my boyfriend has told me many times that any sized boob is a good boob... xD
Anyway, I personally just stay away from push up bras... I mean, there's nothing sexy about your boyfriend grabbing a handful of padding, right?
Apparently if you work out your chest muscles it can give the appearance of bigger boobs... But you won't actually make them bigger as they are just fat... As others have said, putting on weight could help. But then, you may just put weight on your stomach or legs and find it doesn't go anywhere near your boobs. (I have that problem...)