The causes of breast enlargement in boys are divided into obesity, transient enlargement or pathological enlargement. 1. Obesity: generally due to the growth and development process, the body type of obesity, when the weight increases, the fat will also accumulate in the chest, causing the illusion of breast enlargement, but in fact it is the fat caused by the enlargement rather than the mammary gland enlargement. 2. Transient enlargement: boys in the process of development, especially the peak period of development, prone to hormonal disorders, such as estrogen and androgen ratio imbalance, resulting in breast enlargement, but with the gradual growth of the hormone level will return to normal, the size of the breasts back to normal. 3. Pathological enlargement: If the estrogen level is too high, pituitary gland disease, liver function abnormality or the adverse reaction of certain drugs can lead to the increase of estrogen secretion, causing breast enlargement. It is recommended to consult a doctor in time to clarify the cause of the disease and carry out appropriate treatment according to the cause of the disease.