A large overjet occurs when the upper front teeth protrude significantly beyond the lower front teeth. Lip biting habit applies continuous pressure on the anterior teeth, particularly the upper incisors, pushing them labially and increasing the overjet. Unlike tongue thrusting or mouth breathing, which mainly affect other malocclusions, lip biting directly forces teeth forward. Bruxism primarily causes wear rather than dental protrusion. Therefore, the mechanical force from lip biting is a key contributor to developing a large overjet.

Reference: Contemporary Orthodontics, 6th Edition.