The masseter muscles are strong thick muscles located on both sides of your face.  Some people are genetically born with very strong bulky masseter muscles and some peoples get bulkier over time due to overactivity of the muscle.  This overactivity from grinding your teeth can lead to pain in the jaw, headaches and neck pain.

 

A bulky masseter muscle can give a wide, square appearance of the face.  Injecting Botulinum toxin into this muscle can slim the jawline and lower part of the face, as well as relieving pain from teeth grinding.