Peacocking.

Just like long hair (which in humans grows much longer than in any other animal), an ample bossom displays to the world that you are healthy and well fed. Same goes for peacock feathers and lots of other unneccesarily overblown body features.
When you're malnurished or sick, it shows in your body.

Males want women who bear children that will survive to pass on their genes, females want men who will bring the most food on the table and other luxuries that ensure she can raise her children to survive and pass on their genes.
And then it's again evolution: As a result, the most healthy males get the first pick and want to chose the most healthy females. Which means you get children with healthy fathers and mothers, and children with sickly fathers and mothers. The healthy children survive, the sickly ones don't. Which means that genes that make the body look very healthy (long hair, big breasts) can continue, and those genes that make a body look sickly die out. Repeat this process for thousands of generations and you end up with rediculous long hair and big boobs.