Any muscle in the body that is overused will hypertrophy, and increase in mass and size to meet its new demand. Patients with hypertrophic masseter muscle, which lies on the side of your jawline, bilaterally, will have a square looking jaw, and the masseter will feel rock hard when you bite down on your back teeth. Using Botox to treat the hyperactive muscle will slenderize the jaw line and soften the face contour, giving the face a more heart shaped, feminine and symmetrical look