The left breast of adult men is smaller than the right one, which means that both sides of the chest are asymmetrical, and it is a manifestation of the abnormal physical condition, and the reasons for the corresponding condition include one side of the mammary gland hyperplasia, local tumors, local swelling, or collapse of the chest wall, and so on. 1. Mammary gland hyperplasia: abnormal liver function in men may cause estrogen level to rise in the body, resulting in mammary gland hyperplasia on one side, which will cause corresponding symptoms. 2. Local tumor: one side of the chest wall or soft tissue tumors, will cause enlargement of the corresponding conditions. 3. Local swelling: it is closely related to trauma or infection, which will cause local edema and lead to corresponding conditions. 4. Chest wall collapse: congenital developmental deformity or bone fracture will cause one side of the chest wall to collapse, resulting in corresponding conditions. There are other reasons why the left chest of adult men is smaller than the right chest, which require timely medical observation and standardized treatment under the guidance of clinicians.