Iridehorses said it best. Obviously the build and athletisism in a horse plays a huge role. But it's cow-sense that is so deeply inbedded in Quarter horses.
My old cutting trainer kept the babies away from the cattle until they were ready to be put under saddle. One day he was lounging a 2 year old filly and there was a herd across the street. That horse stopped dead in her tracks, ears forward and locked a stare with those cows.
It was amazing to see just how intense these animals are. Yeah, they need training to help them keep off the cow, guid them through their turns, and cut into the herd, but check this video out!!!