Human Growth Hormone (hGH).
They can do either one, depending on what hormone you are talking about, if it is functioning correctly, and the stage of your life.
The hormones that most dramatically affect breast growth are estrogens.
After researching this myself I have found that the # 1 growth stimulator is Puberty. This is the time of ones life where hormones begin the journey into adulthood. Breast development is an early...
Insulin-like growth factor 1 (IGF-1)