Wide jaws can be caused by the shape of bones or muscles, or even can be caused by a mixture of both. 

The muscle that causes a wide jaw is the overactive masseter muscles. They are located in the sides of your face, they are responsible for allowing you to open or close your mouth. The masseter muscles grow in size when you are chewing or biting things, which can influence the shape of your jaw.

Teeth grinding, or bruxism when you are sleeping can also affect the development of the shape of your jaw.

The main bone that can influence the shape of your jaw is the mandible, it is the bone in your lower jaw area, which is connected to the masseter muscle, pulling the jaw to close the mouth. This factor of wide jaws usually depends on the genetic reason, but can also affected by some medical conditions or injuries, which can influence the growth or change of the mandible.