What makes her so fast?

I have a little paint mare. She's just barely 15 hands and a bit stubby and stock, yet she is somehow one of the fastest horses at my barn. She manages to keep up with a huge 18.1 draft/tb cross trotting, cantering, or galloping. She also can jump well over 3 feet. What is it about smaller horses that make them so powerful? It just amazes me, watching/feeling the speed and power in such a stubby little horse.