Are breasts under a large B likely to be less shapely than C or D-cups? In my opinion this is why many men prefer big boobs, because they have more room to be shapely in the first place.
Men prefer big boobs because boobs are one of the few characteristics that differentiate the two sexes, and one of fewer that can be clearly seen without being nude. It's also why women DON'T like men with big tits.