If you gnash or clench your teeth at night, this might explain why your chewing muscles are so big. BOTOX masseter muscle injections are generally done cosmetically to slightly reduce the profile of the masseter muscle on the side of the face. However, this does not really address the clenching. It may be that you would benefit from a night guard to place your jaw muscles in a position of rest at night.
Web reference: http://www.lidlift.com/fillers/