Wagtail
Horse servant
Two things spring to mind. Firstly you're long in the body and short in the legs so if you look at your legs on the horse you're fine but your top half will always look taller - whatever the size of the horse. Secondly who measured him and how because if you're 5'1" he doesn't look anything like 15 hands.
The horse will have no trouble carrying you. However, I agree with jemima that if you are 5'1 the horse doesn't look 15.1. You are 15.1 and in that picture look a lot taller than the horse. Measure him again and see what you get. Even if he is smaller he'll still carry you though.
I agree. From that picture you look about a head taller than the horse. At 5'1 the top of your head would be level with its withers. So either you have grown or the horse is smaller than you think. However, you are not too big for it IMO.