Our data support the thesis that mammotropic hormones, particularly estrogen, are harmful because they suppress the immune system. Heavy women with high levels of body fat produce more estrogen and, therefore, bigger breasts. However, more estrogen leads to an increase in breast asymmetry. Women with “good genes” are able to produce symmetric breasts despite the presence of large quantities of estrogen. Large and symmetric breasts are therefore honest signals of high phenotypic quality in women.
The first signs of breast growth begin as a girl reaches her teens. The fat in the connective tissues starts to accumulate when the ovaries begin to secrete (express) estrogen. The breasts will then grow. Also, the duct system begins to grow. These breast changes often occur at the same time as pubic hair and armpit growth.
After menstruation and ovulation, maturing begins in the breasts with the formation of secretory glands near the milk ducts. With the development of many glands, lobules, and breasts that continue to mature and grow, the duct system and breasts also continue to develop. Each woman's breast growth rate is different.
In the end it's estrogen and the amount of fat that accumulate in the breast, that are largely responsible for the size of the breasts.