User blog comment:Monkey.D.Me/Chibi Luffy/@comment-4498721-20111104220837

I don't think it's about how powerful the attack is or how much stress it causes on Luffy. It was just because Luffy hadn't mastered on returning to normal size, I think.

Maybe we will see Chibi Luffy when he is distracted and couldn't turn back normally or something, IDK lol :D