Large breasts are related to hormone levels of estrogen, progesterone and lactogen. Women with high levels of sex hormones usually have relatively larger breasts compared to women with average levels of sex hormones, most notably estrogen levels. When a woman is pregnant, the level of progesterone will gradually increase, and the breasts will grow and develop further. In the middle and late stages of pregnancy, or after delivery, the level of prolactin will increase, the glands will expand, lactation will occur, and the appearance of the breasts will appear to be nearly double or even more than the normal level. Of course, the size of the breast is not simply related to the hormone level, and genetics, obesity and so on have a certain relationship.