The masseter muscles are the group of facial muscles that are most often targeted with facial slimming treatments. The muscles are located on either side of the jaw, and are involved heavily in chewing food. Depending on diet, genetics and other factors, the masseter muscles can grow very large relative to the rest of the face, creating a muscular and boxy appearance. Some men appreciate this look, but many women are instead frustrated by it. When the muscles are forced into a relaxed stage for a period of time using BOTOX, they shrink back to a leaner size.