Genetics most likely plays a role but how much? My mother was large at about a D cup, two sisters just like Mom and another sister . . . Fried eggs. Go figure. I was on a high dose estrogen & progesterone for 6 months with breast development just really starting towards the end of the 6 months and maybe an A cup, the hormones and my source ran out. This year, 27 years later, I re-started on low dose Estradiol, Progesterone and Spiro and breast growth was almost immediate. Pretty surprising to me since now it's low dose and I had more growth in the first month alone than I did 27 years ago in 6 months. Genetics? Dunno maybe.