Various things. To give a good answer, more information is needed. For example, pregnancy is a cause of an enlarged uterus;however i will assume this is in the setting of being nonpregnant. The uterus can be larger because of noncancerous growths like fibroids or endometriosis, cancer, or a thicker lining from hormone irregularities. Some people can just have a larger uterus than "normal". Hope this helps some.