I'll play your game, if only to try to prove how wrong you are. I weigh 275, but at the beginning of the summer I weighed 290. I ride in an orthoflex cross country saddle that weighs about 25 lbs and I do competitive trail. I ride a 15 hand haflinger that weighs about 1200 lbs.
How do I know that my horse can carry me with no issues? Because there are very stringent vetting rules at every competitive trail ride I go to, where my horse is looked at by a vet for lameness, muscle soreness, and metabolic function, both before and after each ride, and my horse scores 99.5's, meaning that after carrying me 15 miles in 2.5 hours, he has not changed at all. If he acted at all like he was tired after 15 miles, I might consider riding him less or conditioning him more, but he is ready and willing to go farther at the end of our rides.
Before pregnancy I was 168 (actually I dropped weight at the beginning and am now back at this per my last dr visit) at 5'4". Based on BMI I am darn near "obese"...but have yet to EVER get told by a doctor that I need to lose weight- plus I have been poked and proded and found to be very healthy and actually at my last dr visit I was complimented on my abs (6 months prego)!
I guess Im just like a good tomatoe- heavy for my size
My main horse is a 14.1 966lb (taped on monday by the vet) qh mare. I would guess that all of my equipment is roughly around 50lbs.... When Im in more riding appropriate shape That horse gets ridden pretty hard with out a problem. Trail rides, sortings ect and she goes ALL DAY just fine.
IMO that stupid 20% is just another way to bash on fat people....I can garantee you that alot of those slender professional trainers you see riding these young and or small horses come in at close to 200#s...and you don't hear jack about this 20 % rule non sense. Every horse is an individual and should be treated as such, some slender built horses can carry alot of weight and go all day, where some other even hardier built horses can't..but I also believe that comes down to conditioning as well.
I weight 145lbs about 5'9" and ride a 1250lb QH and a 800lb grade welsh without any problems.
The QH has a bigger dislike for heavier riders in comparison for his size, he thinks that I'm the right size and anything much bigger is just asking too much. He is much more forgiving of beginner riders (on property) if they are lighter and much less forgiving of a heavier rider if they are unbalanced.
The pony can and has gone all day with me on her back with no lagging or issues keeping up. Not just walking around either, trotting, cantering, galloping, running through water, up and down hills, and racing a couple of QHs. She's a willing little creature and very honest, but sensitive enough that I have no doubt she'd give me a lot of hell if I was too much for her.
It depends on the horse, like so many things in the horse world. A rider must learn to read their particular animal and determine for themselves what they are capable of doing. For the most part I think "heavier" riders are far more aware of their horse's limitations than lighter riders. I know when I ride the pony I'm watching for signs of me asking too much more than when I ride the QH.
So not playing, but as to the weight carrying thing...
I am not a huge fan of using the 20% rule as gospel, maybe a good guideline.
Some common sense thoughts:
If 20% is the guideline for a horse carrying a man all day every day in the cavalry, do the same guidelines apply to cross country, a days hunting, a half hour stroll at a walk?
Emmy is currently overweight, if I taped her and worked out 20% it would not be a true reflection on her weight carrying ability.
I like this calculation, as it takes into consideration the horses bone, and the combined weight of horse and rider
Add up the total weight of the horse, rider, and tack. Our example: Horse + rider + tack= 1188 pounds
Measure the circumference of the cannon bone midway between the knee and fetlock. Our example: 7.5 inches
Divide the total weight by the circumference. Our example: 1188 / 7.5 = 158.4
Divide the result by two. Our example: 158.4 / 2 = 79.2
Values below 75 are great! Values from 75-80 are acceptable. Values over 80 indicate weaker legs and a need to train carefully, especially downhill. At this level a rider needs a horse with more substance.
Even with this one of the issues is the ability of people to accurately measure or asses their horses weight, weight tapes are notoriously difficult to use properly.