I do not think it is rude to have this discussion in the plus sized forum. In fact, I actually think it rather apropros, and those that brought it up really need to have quite the backbone to raise an issue that is likely to ruffle some feathers . It's just not PC to these days to say that a person might be too heavey for riding.
However, there IS a limit. We all know that. And, there are some of us who may actually be wearing our horses down faster than they would if we were not as heavy as we are. But, we ride to "use" our horses; for our enjoyment. They have a natural lifespan which is both shortened by our "using them up" and lengthened by our feeding and caring for them properly. IN any case, we use them. It may very well be that the very heavy rider will use up the horse faster than the not so very heavy. It may be that the "normal" person , who rides super hard and over jumps the horse will use it up faster, too.
But there are limits as to what is a reasonable way to "use" a horse, and there is no reason why weight is one that cannot be discussed because it will cause hurt feelings. It is simply another parameter in horse care/health.