It's been over 5 years but if I remember correctly the mc is in a chibi form because he's considered a runt or something and can't turn into a full fledged adult but he's a vampire or succubus/incubus that needs blood from his master. In this one shot he's able to transform for a little while and he had angel wings even though he was a demon(?)
OH! another thing the mc has to eat a certain candy or something, I don't remember if it was as a reward or not. I think this was also a contract between a human and demon(?)
Please help me, I'm getting desperate! I've been searching for 3 hours already.