Can you have a small face and a large jaw line?

Perhaps : I would have to see the patient, but sounds like the upper jaw has not grown as it should have and therefore, possibly causing the lower jaw appear bigger.

Relative: All things pertaining to size in anatomy are relative to other surrounding anatomical structures. A perfectly average size lower jaw may seem large if the skull or the upper jaw are small(er). It is all a matter of proportion. Large is only large relative to the position and function with neighboring bones, etc.