There are many reasons for teeth growing in crooked, overlapping, or twisted manner. The shape of the jaws and facial structure of the mouth varies from person to person. Sometimes, the size of the jaw is too small and the teeth are big, which crowds the teeth and causes crowding. In some cases, the size of a person’s upper and lower jaws are disproportionate or malformed. This results in either an excessive protrusion of the upper jaw or lower jaw.