The possible reasons for the thickness of one leg and the thinness of the other are as follows: 1. Muscle atrophy of the leg: for example, if a fracture occurs in one lower limb and a long-term cast is fixed, the affected limb will be significantly thinner when the cast is removed, which is due to the disuse atrophy of the muscles caused by the lack of activity for a long time. 2. Inflammation: if osteoarthritis of the hip or knee occurs, the patient will often refuse to use the affected limb because of the pain of the joint. In daily life, the patient mainly uses the opposite healthy limb to compensate, there will be muscle growth in the healthy limb and muscle atrophy in the affected limb, so there will be unequal thickness of the two legs.