1) When jumping, a horse's ability depends on the force generated by its back legs and speed. Bigger horses generate more force and are better jumpers than smaller ponies.
2) According to Newton's Second Law, a horse's mass affects the force it can exert for jumping. Jumping form also matters, as riders balance their weight in a two-point position to not impede the horse.
3) Faster gaits like canters provide more force than slower trots, allowing horses to clear higher jumps. The author's experiment found the biggest and fastest horses jumped highest.