The growth and development of breasts are affected by the environment, nutrients, personal constitution and hormone secretion level, etc. Therefore, the causes of small breasts include genetic factors, endocrine disorders, nutritional imbalance and excessive weight loss. 1. Genetic factors: mammary gland development is related to genetic factors, so small breasts are not necessarily caused by pathological reasons. 2. Endocrine disorders: if gonadotropins, lactotropins, estrogen and progesterone secreted by the ovaries are not sufficiently secreted, the mammary glands do not get enough stimulation, which can also lead to small breasts. 3. Nutritional imbalance: If you are undernourished or picky about your eating habits during the growth and development period, it will affect the selective deposition of fatty tissues in the breast area, resulting in flat and small breasts. 4. Excessive weight loss: long-term in a state of hunger, will inevitably mobilize the stored fat and protein to supplement the supply of energy, and the breast is mainly composed of fat and collagen, therefore, excessive weight loss will result in a significant reduction in the breast fat tissue, breasts become smaller and sagging. There may also be other reasons for small breasts, such as congenital dysplasia of the mammary glands. It is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or treatment.