i don't think there is much problem, inless the horse has some medical condition, also may i ask that you use smaller text and change the color, it really hurts the eyes and makes it difficult to read.
Anyway's i think you have nothing to worry about, and if you called the RSPCA, they will probably laugh at you.
I also think that this is offending to overweight riders. Im just saying that just because the horse is 15 HH, and the rider is overweight doesn't mean that the horse is in agony or pain.
I know alot of 'overweight' riders and they are perfectly fine... these animals have strong backs and are large, considering he is a cob also.
Horses can pull carts and wagons also, just going to say again, you don't have anything to worry about.... most likely i dont see a problem with it.