Is estrogen the direct link to bigger breasts on a female? is it possible for her breasts to get bigger if treated with estrogen? if not, how?

The "direct link": Is the genetic information you inherited from your parents/ it is the primary determinator for your breast size. Estrogen, such as bcp's, can increase breast size, but only while you are taking it -- such as contraception or pregnancy. Why do you need to increase your breast size in the first place -- remember it's quality, not quantity.