Called micrognathia, a deformity of failure of the lower jaw to grow.
It can have genetic and syndromal reasons. If just one side, it is called hemifacial micrognathia. Truama at an early age can be implicated as well. Maxillofacial surgeons can advance the lower jaw, and if that is not enough, they can fashion bone grafting from rib or pelvic bone to add to the body of the mandible.