A flat chested girl with a plump round ass is perfectly fine, a heavy chested girl with an ass like an ironing board is not something I'd want to fuck more than once. I love boobs but the importance of the butt surpasses that of the boob where longevity is concerned in whatever relationship there might be between you and her, whether it's just sexual or more.
Tig bitties but an ass like an Auschwitz survivor just looks wrong.