It's really a personal taste thing, rather than "good vs bad". Just as some people like blonds more than brunettes and vice versa, some like 'em big, some like 'em small.
Me, I'm not picky. I think curvy girls are gorgeous, but there's something to be said for the smaller-chested ones too, especially if they're a bit androgynous to start with. In the end, for me, it's more about overall appearance than size. If they look really fake, that's a big turn-off for me, regardless of how big they are.
(Bisexual male speaking, for the record.)