An oval face is often sought after as it symbolizes youthful femininity. A square face or jaw line is often due to the overdevelopment of the chewing muscle (Masseter Muscle).
This muscle overdevelopment can be genetic, or due to excessive chewing (e.g. chewing gums) or teeth-grinding (Bruxism) when asleep. Often, one will notice the face getting more square or wider with time as the muscles gain strength and bulk out through the years.
Botox is an effective treatment method for reducing the muscle bulk of the chewing muscle and slimming down the jaw line. By reducing the strength of the chewing muscle, the bulkiness of the muscle will be decreased - just like banning a body-builder from the gym will make his muscle mass decrease.
To see if someone is a good candidate for using Botox to slim down a square face, he or she is asked to clench his or her teeth and if masseter muscle overdevelopment is present, it will be felt as a hard bulk.