A leg thick a leg thin consider physiological factors, lower extremity venous thrombosis, malignant lymphoma and other factors. 1. Physiological factors: athletes or one side of the limb is used too much of the population, due to one side of the limb muscle than the opposite side of the relatively developed, long-term will appear a leg thick a leg of the phenomenon, generally belong to the normal situation. 2. Lower extremity venous thrombosis: venous thrombosis causes one side of the limb blood reflux obstruction, resulting in leg and foot edema, swelling, and the phenomenon of one leg thicker than the other. 3. Malignant lymphoma: one side of the limb lymphoma swelling compression of blood vessels or tumor thrombus blockage of blood vessels can also lead to the phenomenon of one leg thick and one leg thin. There may be other reasons such as varicose veins in the lower limbs. It is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease and then give targeted treatment or therapy.