I saw an interesting program on Discovery or History channel once and they talked about this very issue.Although they didn't have an exact reason,they had an interesting theory.
It all relates to re-production(doesn't everything?).Women with curvy full hips and butts are naturally seen by males as desirable partners for mating because these features normally aid in childbirth.
They showed a side by side pic of a woman's breast cleavage right next to a woman's butt cleavage and you couldn't tell the difference.I guess a nice deep cleavage and full roundness,front or back,sends a message to the brain to mate.
On another note,the breast size and shape is only half the equation.The nipples can really make or break a nice set of breasts.