A wide or heavy-set jaw can often be caused by enlargement of the masseter muscles. The masseters sit over the jaw and can be felt when clenching the back teeth. In some people, due to genetics or frequent jaw clenching, the masseters can enlarge. Masseter enlargement can make the lower part of the face look wider and more masculine. Botulinum toxin or ‘Botox’ injections can be used to safely and effectively shrink the size of the masseter muscles, reducing the width of the jaw and making the lower face look slimmer.