If he has a good barrel then I shouldn't think so. I'm 5ft 8 riding a 15hh and I don't look too bad. As long as you are not too heavy it should be fine. Ponies are waaaay more fun, lucky you for being so small!
I was going to post something similar. I am paranoid about being 5"5 (9st12) on a 14h.
To be honest if they can carry you then it's ok. I only worry in case I look silly and he looks squishable! I am going to get my OH to take a picture of me on him - maybe you could too?
its just that all the nice sensible ponies are about 13.2! i dont mind loosing a couple of inches if they make up for it in gentlemanly (or ladylike!) manners
I'm 5ft9 and ride my 13.2hh....though that said I've put him up for sale :/ He carries me fine but obviously look ridiculous on him proportionwise lol.