Genetics play a very important role role in breast development. If the women in your family are small breasted, more than likely you will be small breasted. In a similar note, the amount of estrogen receptors your body has, also plays a big role in breast development. I've know some who were maxed out on estrogen dosage, who had little to no breast development, and conversely I've known some who have had significant breast development while being on low dosages of estrogen (this is the category I fell into - I never took more than a half dose of Premarin, or [when I switched to it] a quarter dose of Estradiol). It is also time dependent, the longer you're on HRT (up to a point - after a certain amount of time, it will stop), the more development you will have. I remember reading somewhere that it can take as long as 10 years for even ciswomen to reach maximum breast development.