Harvard anthropologist, Frank Marlowe, believes that breasts have been used in the past for their ability to indicate a woman’s age, and hence her reproductive abilities. Large breasts tend to be heavier and sag with age. With smaller breasts, this sagging is less apparent, so it’s harder to tell how old she is just by looking at her. So men have evolved to examine the breasts, and prefer large ones.
Others believe large breasts indicate other forms reproductive ability. Two reproductive hormones, estradiol, and progesterone are found in higher levels in women with large breasts and tight waists. So men have evolved over time to prefer large breasts.