i'm 5 ft 7, 8.5 stone (maybe 9 now) and regularly jump and ride a 13hh fine pony, and a 13 2hh slightly chunkier cobby pony. both carry me well.
i wouldnt ride the 13hh pony till she was dripping with sweat, and i always get a leg up on her so as not to put too much weight on her back trying to get on