It has to be big and strong enough but light enough to be athletic. Most pure drafts are too heavy. You see lots of draft crosses and heavier boned stock breeds. You see lots of spoiled saddle horses go through bucking auctions, qh type, paints and appys with decent bone, athletic and with the desire to get rid of their riders.
One of the most tallented bucking horses I'd ever seen(not at a rodeo) was a spoiled Quarter horse.