Last week, I rode a 14hh mustang mare. This week, I rode a 18hh thoroughbred gelding. I love the impulsion of the thoroughbred, and his ground covering stride. But at 5'11", I will say I had a lot
of fun on that little mustang mare. I'm not afraid of the little mare, and the ground is a hell of a lot closer if she decided to duck out from under me over a jump!
Now, that 18hh tall gelding and his HUGE impulsion, I am actually a little bit afraid of jumping him. I know he will have a ginormous jump, and I don't know if I will be able to stay on it.
I can imagine his huge leap, with his long legs, and his arching back, tossing me right off over even a short jump. I know it will be huge. I rode a 16.2hh Appy that had a big jump, and it took serious effort to stay on him when he over jumped. So I can say I am actually afraid of that 18hh'er and the LONG way down to the ground, even though I know he wouldn't do anything to hurt me. He won't even pick up his butt high enough to give me a decent crow hop.
I "fit" a 16.2hh horse or bigger; I can sit the longer stride and impulsion really well, and I have the leg to stay on if they decide to get funky. However, I started riding on 15.2hh horses, and I spent a lot of time on dinky little QH's, short Walking horses, and a few shorter than 14hh ponies, so I am very comfortable on a short horse, even if my feet reach their knees.
It looks silly as all get out, but I ALWAYS end up riding a pony, just because they're so much fun to ride.
As far as I know, western disciplines don't care so much if you ride a shorter horse, mostly because QH's aren't always that big, and supposedly western riders sit with a really long stirrup. It's really the dressage ring where they seriously care about the length and the aesthetic "value" of matching heights.
So no, I don't think you have outgrown your pony. Your feet aren't to his knees, and so long as you can still give him clear leg signals, he will never be "to small" for you.
Good ponies are worth twice their weight in gold. Keep him.