Depends, I think it's more about symmetry... Boobies completely out of shape or oversized aren't really so attracting. Bigger women can have bigger body parts without being "fat" compared to smaller women. Smaller boobies have benefit that they tend to be more sensitive than bigger ones, if you like to play with them. But to be honest, if I really preferred flat chested women, I'd be topless and fingering myself next to mirror... Boobies are kind of one of the signs of womanity.