For the same reason I got a big dig and you have innie bellybutton for a penis. God doesn't love you.

breasts grow not just during puberty, but also during courting, sex, and pregnancy -- unless the womans biosystems are screwed up (these days they often are due to drugs, birth control, pollutants, etc)

Fun Fact that most people do not know is that the larger breast size is for reproduction in modern times. In times long past before civilization women all had small breasts to be able to run from predators when not pregnant and be able to climb and run faster but when pregnant the males would protect them. Now in times with civilization and cities there came food excesses but some women had a tendecy for a higher number of fat cells and uglier features like big skulls or round faces and men would not be attracted to them fat women with small breasts. These women died off quickly but some womens ancestors bodies evolved the larger breasts imitating the pregnant womens one and men would naturally be fooled by them and stay near like a protective instinct and end up really getting them pregnant since a man that protects a woman that is not his family has a natural tendency like any male in an alpha role to take sex as payment for protection or care of a female as does any animal. The smaller breasts on the other hand often had more beautiful faces and healthier sexier defined bodies did not need the larger breast or as the smaller women call them hanging fat bags to attract a mate under normal conditions.

Because women are are silly bastards