Theodore O'Connor was a pony! 14.1 3/4 or something. Most incredible little guy.
Jack of Diamonds was a GP showjumper wayyyyy back when. 14.2hh.
There are tons of accounts of ponies jumping higher than their own height. 12.2hh ponies jumping 1m30 [that's 4'4"-ish!], Connemaras winning at puissance against 17.3hh jumping-bred warmbloods, the list goes on. Ponies can be awesome!
Edit; and way back in the days of "high-jumping" competitions, before WW2, it wasn't unheard of for 13hh ponies to jump 7 foot high fences!