Well you see, It is a little somthing called puberty. When females begin puberty,( puberty can begin anywhere from age 9 to age 15) he horomone estrogen is realeased in the females body, which develops the female sexual organs. When females grow breasts, a small bud will grow underneath the boob to be(fittingly called breast buds) and evaentually throughout the course of puberty grow into breasts. Usually, one breast will be fully developed beore the other, so females should not be concerened if their breasts seem lopsided at first(in fact, they will most likely catch the attention of more males). So, the significant difference in Mia's mamories is perfectly explainable by nature.