Personally, I don't think you are too big. I do prefer a nice 12hh stout pony myself (at 5'7"). My girl is 13.2hh and I got to ride her the other day and she is absolutely everything I could want. Now, I'm not into showing and the showing crowd has their fashions (colors, height, breed etc) but if he can do what you want without being in pain and you like him, then I say by all means keep him. I personally think that the 5 foot nothing people who think they need the 17hh horses look far more ridiculous than you do on your guy!