I am currently just under 13 1/2 stone. I am 5'9". Skippy was 16.3 idx and carried me at my heaviest (14.3) without batting an eyelash. Arnie unfortunately, didnt make the height I hoped for so is just 15hh (with his shoes on!). When he was backed, I had lost weight and was 13stone. He carried me fine at that. He also carried me fine at 14stone but, I didnt carry me! At 14 stone, I struggle to get on and off. I feel like a sack of spuds on him. I get short of breath and bounce around on his back. I certainly wont jump him at that weight.
Wee slim Lucyad on here is taking him to his first XC on sunday. There is no way on this earth I would do XC till I have got a bit more weight off. I dont mean vast amounts here, if I get to 13 stone thats great.
I think the point a lot of us are missing here is, its not fair on the horse unless we are fit! Gentle hacking is absolutely fine and perhaps even a short lesson but fast work and jumping is unfair unless we are fit enough to do it - and the bottom line is, most of us who are overweight are also underfit. I would have no problems letting a fit, active 15 stone man ride Arnie. I would have a problem letting an unfit 13 stone ride him. For me, its not about the weight, its about the fitness.
The other big issue I have is with overweight people squashed into too small a saddle. I hate to see a riders weight all on the cantle. That is concentrating the weight in a small area and is really unfair on the horse.
A bigger, fitter rider in a well fitting saddle should have no major problems for general day to day riding.