Breasts appear one big and one small reasons are as follows: 1, congenital causes: women in puberty, the development of breasts of different sizes, may be in the embryonic period of various reasons; 2, traumatic causes: childhood trauma will affect the mammary gland development; 3, lactation causes: the most common reason, due to the problem of lactation drainage, the formation of mammary cysts or mastitis; 4, the cause of the acquired disease: such as mammary tumor, mammary cystic hyperplasia, etc.; 5, occupational reasons: may be engaged in a long period of time in an occupation caused by one side of the chest muscle is more developed The most common reason is the formation of breast cysts or mastitis due to the problem of milk excretion; 4. Acquired diseases: such as breast tumors, breast cystic hyperplasia, etc.; 5. If the size is different, it is recommended to do ultrasound, it does not affect the function, and there is no uncomfortable symptoms, you can not deal with it.