I've always thought that the power of his keyblade and the light of his heart are what give him the strength and ability through the magic of the keyblade, hence "the keyblade chooses its master" it sort of becomes one with its wielder. That's where he gets his strength and crazy moves from, not muscles. Sora is an underdog hero, he's 5 ft tall (literally, game model measurements), skinny, dorky and clumsy, wears funny clothes and has a kind heart. If he was tall and buff it just wouldn't be the same.