Ahem, as one of those "skinny" riders (actually athletic, but a lot of people use the term pretty loosely around here
), I would say you are DEF not too big to ride!! Not even sure where you got that notion, you look just fine to me. If you want to loose weight then that is up to you, if your horse shows no discomfort and his back and joints are just fine then I see no reason to worry. If you are THAT uncomfortable, then just get a bigger horse, but believe me girl, there is nothing that should keep you from riding and enjoying what you love to do.