Hi, and thanks for your question!
The size and shape of breasts is mostly a result of genes and inheritance from both the mother’s and the father’s side. So even though they are inherited, mothers and daughters can have very different breast sizes as the genes from the mum and the dad influence the size.
Another factor that has an impact on the size is how much body fat you have, as breasts are made up of a lot of fat. So slim girls will have smaller breasts than plump girls.