Here's some info for all of us to chew on according to Dr. Deb Bennett:
The basic rules and concepts are:
1. Total weight of rider plus tack must not exceed 250 lbs. There is no horse alive, of any breed, any build, anywhere, that can go more than a few minutes with more weight on its back than this. Not even the U.S. Army ever packed a mule heavier than this. So if you mean by "plus sized riders" that they weigh more than 250 lbs., including the weight of their saddle, then in good conscience what you must tell them is to look for another hobby activity, or confine their horse activities to either driving or groundwork.
2. Horseback riding is not a right: it is not a right for small people, not a right for large people. It is a privilege. If the person is too big to be on a horse's back without doing the animal harm, they should in ethics and courtesy acknowledge that as a fact -- not demand that they ride anyway. To do this would be like demanding that, because they dream of being Superman, they should be permitted to fly when they dress up in a red cape and jump out of a fifth-story window. In other words: what they dream of is a violation of natural law, to which there are no exceptions.
3. It is a lie to tell any child that he can be anything he wants to be. You cannot be anything you want to be. You can only be what you are fitted to be. If you have a tin ear, I don't care how much you practice; you are never going to be Mozart, and you are also never going to be pleasant to be around when you are practicing your clarinet or your voice lessons. If you are short, you can love basketball with all your heart and learn to carry the ball aggressively and shoot like a demon, but you still aren't going to get on a professional basketball team. If you are long-limbed and not very strong, you're never going to be a gymnast. Unless you have knees that bend both directions, you're never going to be an Olympic medalist in the butterfly or freestyle. And unless you weigh less than 250 lbs. including your tack, you will struggle on horseback, you will fall more often, you will get hurt worse every time you do fall, and you will slowly ruin your horse's back. If you are a plus-sized person, find an activity that you like and one that you have an aptitude for.
4. A "plus sized person" is not one that is necessarily overweight. However, many large people are overweight. The more anyone is overweight, even if they have talent for horseback riding, the more they limit themselves. I know this from my own case. A decade ago, before my knees became so painful that I could no longer run (except in a therapy pool), I weighed 20 lbs. less than now. I am aware that the extra 20 lbs. costs me in the areas of stamina, flexibility, timing, balance, and feel, both in ground schooling and in the saddle. Another way to put this is that for every extra 20 lbs. you're carrying, you lose 1 "dressage level". I know several obese upper-level riders who would be KILLER if they would or could lose weight. Sometimes, for whatever reason, weight loss isn't going to be possible, and if that's the reality, and the total is above 250 lbs. rider + tack, then the person needs to re-structure their horse activities in such a way that they stay off the animal's back.
5. The maximum weight-bearing capacity of 250 lbs. already mentioned applies only to horses with weight-carrying conformation to their backs -- and this by no means includes all horses. Some horses cannot comfortably carry even 175 lbs. rider + tack; so that if the rider rides 'western' where the saddle typically weighs at least 20 lbs., then the rider is going to have to be below around 155 lbs. Horses that cannot pack the maximum have long backs and, more specifically, narrow, tubular couplings -- they are what we call 'dog backed' because the span from the last rib to the hips is rounded and narrow as in a dog. We do not ride dogs and this is the essential reason why. To be a weight-carrier, a horse needs a strong loin coupling, which is short, smooth, pathology-free, broad, and deep. See my conformation series in Equus Magazine, the issue out this month, for helpful specifics on the coupling.
6. The ability to pack weight does not increase with height. Tall horses in fact often have weaker backs than shorter ones. Ponies often have the strongest backs of all. It is a question of absolute breadth across the loins, secondarily of depth from loin to groin, which indicates whether the horse makes the effort to arch its back and oppose the rider's weight with every step he takes.
7. The ability to pack weight does not relate specifically to the size of the feet or the amount of "bone". If you have been following the Equus Magazine series, you will already have learned that the heavier the horse (or horse + rider + tack), the more "bone" the horse needs in order to stay sound over the long haul. However, to be a weight-carrier the horse always has to have the short, broad coupling. If he is long-backed and dog-loined, he can have the recommended amount of 8 inches or more of bone-tendon circumference per 1,000 lbs. of weight, and it won't help him: his back will still go down over time, the weaker the sooner.
8. The ability to pack weight does relate heavily to how well the horse carries himself. Many, many horses ridden by people plenty small and light enough to fit below the 250-lb. cutoff weight slowly ruin their horses over time anyway, because they either do not know how, or do not care, to insist that the horse round itself up the entire time they're in the saddle. "Rounding up" is the minimum requisite effort that fills the rider's seat and opposes and neutralizes the downward force of her weight.
9. The ability to pack weight comfortably over the long haul also relates heavily to riding technique. If the rider bounces when trying to sit the trot, intentionally jabs the horse with her seatbones, sits all the time to one side of the saddle, hangs on the reins, or believes in "breaking the horse back at the root of the neck" in order to obtain high head-carriage, these things are recipes for a short useful life for the horse, and again, the weaker the back to begin with, the sooner it will go down.
10. The exact same may be said of tack: the heavier the rider, the more crucial it is that the saddle fit the horse well. And the more inexperienced or clumsy the rider, again the more crucial is good saddle fit. A rider's mere presence on the horse's back hyperstimulates the muscles that HOLLOW the back; so the last thing we want to do is add to this by hyperstimulating them further through bouncing on his back and/or jabbing it with an ill-fitting saddle.
These are the truths that I can share with you concerning rider weight and the horse's weight-bearing abilities. I am not lecturing anybody to lose weight. What I am doing is reminding you that not everyone is cut out to be a rider; there is a limit above which we cannot go and still claim that we care about horses' welfare. There are plenty of satisfying high-skill activities around horses other than riding, which makes it even more wrong to use these beautiful animals as vehicles to fulfill an unattainable fantasy. -- Dr. Deb