Hm. I have fallen off a 16.2 hh Paint. That hurt. A lot. But he threw me against a rail. Now, another time he just bucked me straight to the ground, and that didn't hurt as much. Not as much as when I forgot my helmet, and fell off my 14.3hh pony at a canter. So really? I think it just depends.
I've only technically fallen off my 17hh once, and we kinda fell ... together... so it was more like scooting off a 5hh pony. So?? Idk XD