It sounds like she's actually a good height and weight for that type of fighting. Some Asian kickboxers are considerably shorter than that and not much heavier. Bruce Lee was in the 120-135 lb. range but he could hit as hard as someone a lot bigger because he got his whole body into it. A key for a smaller person to be good at martial arts is to perfect their whole-body coordination so that their whole mass is lined up behind their punches and kicks. This basically means keeping the center of gravity low with the knees well bent and good posture, keeping the elbows and knees lined up in a way that the body supports them when punching or kicking, and coordinating the waist with any turning movements. Some girls can actually learn this better than boys because 1) they have to in order to get any power into it and 2) they have a more intuitive feel for their bodies.