Is it possible for people with small lower jaws to have an underbite?

Unlikely: A class 3 bite, where the anterior teeth are in a crossbite, is usually due to an arch size discrepancy where the lower jaw and arch form is diproportionately larger than the maxilla.

Yes, but not likely.: It is possible but not very likely. Most people with small lower jaws will be predisposed to have excessive overbite and overjet.

Nope: An underbite "negative overjet) means that the lower jaw is more forward than the upper jaw. So the lower jaw would necessarily have to be larger than the upper jaw.

Unlikely: The key here is one of definition. An underbite typically indicates that the lower jaw is forward in position or "larger" than the upper. It's best to have an evaluation with an orthodontic specialist to determine the exact nature of your problem and possible solutions.

Overbite?: If lower jaw too small usually presents as excessive overbite (deep bite, buck teeth, or both). Usually underbite refers to upper jaw too small or lower jaw too large, causing lower teeth to be outside the upper teeth. Why don't you treat yourself to an examination by an orthodontic specialist who will check you out and explain 1) what's wrong, and 2) how it can be fixed.