Our hero. The biggest thing about him is his courage (and his feet)!
For me, I think that there is something fragile and precious about the hearts and minds of young readers, and that is a quality that stays with us and shapes the way we dream up our fantasy worlds and characters. Imagine the world through a child's eyes: seeing the world and the grown-ups and all of their wild, confusing dramas as so much bigger than we are and so terrible in their power that to try to stand against it is almost unthinkable--we'd have to invent a world in which to level the playing field where, as Maurice Sendak might put it, we could be "the most wildest thing of all." (That changes the "innocent/precious" child into a power worth reckoning, gleefully vengeful and benevolent in equal measure.)
As far as wish-fulfillment, Max did it best!
Fantasy is able to mold the world around the smallest persons living among the taller, more powerful people who Make The Rules and allow them to become champions of the tiniest voices lost in the crowd. Whether dolls or faeries, pigs or rabbits or mice (or mice or mice), three unfortunate orphans or the one orphan Boy Who Lived; these miniscule heroes stand up to make a difference, not only for themselves but for their world as a whole and we, the wide-eyed reader, cheer them on because we know what it's like to feel small, we know what it's like to be ignored, we know that quiet, helplessness when things seem Too Big, Too Complicated, Too Scary, Too Difficult for us to understand what to do and so we should just sit quiet and let someone else make the decisions instead of standing up for ourselves and for others.**
But a hero doesn't do that. A hero takes action.
A true hero speaks up. Even if they are only a Very Small Animal asking for help.
We search for that little hero inside all of us and place them in a Big World with Big Problems to prove that it's possible to do what's right no matter how small you are (or feel). Like Mrs. Frisby, you can make a difference, take a stand like Lucy, change the rules like Keladry, Protector of the Small, start a revolution like Katniss Everdeen & even, like Dorothy, find your own way home. There is something immensely powerful in that story: the one where the little guy (or girl) wins. It's something like hope, a little like wishing, and captures the true essence of heroism.
And that is no small thing.
* As opposed to the underdogs who are Big Damn Heroes...but that's another fandom.
** P.S. This feeling never really goes away, of course, even after we grow up.