The reason why the right breast is bigger than the left breast is considered to be the following: 1. It may be caused by the different sensitivity of the breasts to estrogen during the puberty development stage. 2, If the right breast is suffering from breast enlargement, mastitis, or breast cancer, the lump inside the breast will make the whole breast look bigger than the other one. 4.It may be caused by unilateral breastfeeding when breastfeeding. If it is caused by disease factors, it is recommended to go to a hospital breast specialist in time to avoid missing the best time for treatment.