Want to make something cuter? Just make it smaller. Look at kittens and puppies. Don't you just want to hug and squeeze them and call them George? Okay, perhaps not that far, but they are cute. And it's not merely being smaller, but also having a "smoother" shape than adult dogs and cats, and usually having bigger eyes. Of course, the best-known anime and manga method of making something cute is the Super-Deformed look (a.k.a. "Chibi"). The trope's title is from 'Fun-sized' candy bars, the smallest size typically seen as Halloween treats. Has nothing to do with a certain other Fun Size.