Small breasts in girls may be caused by genetic factors, malnutrition, ovarian insufficiency and other factors. 1. Genetic factors: the development of breasts is related to genetic factors. It is generally believed that if the mother’s breasts are small, the likelihood of her daughter’s breasts being small will be high. Half of the human body’s chromosomes come from the mother and the other half from the father, so the father’s chromosomes can also lead to small breasts in girls. 2. Malnutrition: If a girl is thin or malnourished, she may suffer from slow development or nutritional deficiency during the process of development, which may easily lead to the lack of sufficient fat in her breasts, so her breasts develop smaller. 3. Ovarian insufficiency: due to ovarian insufficiency, the ovaries cannot effectively secrete estrogen and progesterone, which affects the development of mammary ducts and mammary follicles, resulting in poor breast development, so the breasts are smaller. Inadequate underwear during development can also lead to small breasts in girls, and patients should promptly go to the hospital to consult a specialized doctor to clarify the cause and actively treat it.