Associate breasts are mostly caused by congenital developmental abnormalities. They are most frequent in the axilla, showing lump-like localized elevation, nipple-like elevation or areola-like pigmentation in the central part, which may be accompanied by pain, and may show asymmetric growth with one side big and one side small. It may also be a disease situation, such as one side of the paratesticular gland hyperplasia, fibroadenoma and other diseases. 1. Normal phenomenon: with the development of puberty, the paratesticular breast gradually swells, and increases significantly during menstruation, pregnancy and breastfeeding than usual, and the phenomenon of one side of the paratesticular breast being bigger and one side being smaller may occur, which is a normal physiological phenomenon. 2. one side of the paratesticular gland hyperplasia: endocrine hormone metabolism imbalance may lead to excessive proliferation of breast tissue, the symptoms of paratesticular breasts on one side of the large and small. 3. Fibroadenoma: trauma or radiation exposure may lead to fibroadenoma, and the side with fibroadenoma may be larger than the normal side. The symptoms of fibroid tumors may also be larger than normal on one side of the breast, and may be caused by breast cancer. If the symptoms of breast cancer persist, you should seek medical attention.