There is another factor nobody is considering, besides the comfort of the horse, and that's the safety of the rider. I have put on a lot of weight and can feel my balance is not as good as at my proper weight. Overweight riders have a much higher center of gravity as the extra weight is disproportionally at or above the waist making a person top-heavy.
Simple physics would demonstrate that person is more likely to be thrown out of the seat with a lurch, bolt or stumble. If you want a feel-good forum that encourages people to be in denial of the scientific evidence the stress caused to horses with overloading, then you should title the forum as such. But if you really want to be safe and keep your horse happy and sound, you should follow the experts and research this rather than let a bunch of strangers saw "awe, you're fine" without any info to base that on other than your weight.
You are not too heavy to ride the appropriate horse at 247. At 5'3' you are probably top-heavy and would need a well trained reliable horse capable of comfortably carrying you and your tack. Most of the time, your horse is not going to complain when you and i would under equivalent pressure, because they are genetically designed to hide and mask pain in order to survive as a preyed upon herd animal. That instinct is alive and well even after centuries of domestication.
I have several thousand dollars of vet bills from a state veterinary hospital proving that irreparable damage can exist in a horse who was injured from overloading, even when a well respected trainer, the experts who frequent his barn, my own vet and a vet chiro could not find an issue. The horse demonstrated no obvious signs of pain, she only refused to take her right lead in training. That was the only clue. She has an irrepairable stifle stress injury she will have for life. I spent a year and a half trying to make her sound. None of the experts thought she was in pain. Your average horse owner would not have had a clue. Do you really know if your horse is suffering? Not likely, if they are not displaying signs.
Also, i have a crushed knee from a top-heavy related unseating when my well trained (and appropriately sized) qh stumbled at a lope. As a former barrel racer, and avid trail rider including competitive trail, i had never been unseated in my whole life. It was like i had a sack of feed strapped to my back when the inertia took over. i am a strong legged muscuar person. Now i have a disabled knee that causes me daily pain.
So i am a little put off by those in this forum who are saying it's fine to overload a horse. Please publish proof before you encourage someone into danger and injury for both horse and rider.
I completely understand where you are coming from but:
1. The 20% thing is a guideline not a rule, every horse is different and most often with a dedicated owner or trainer they can judge the abilities or discomfort (if any). Would I want to push it to the max and see if it was okay...probably not. Do I feel bad about putting someone overweight on my stocky QH...nope. Its all about personal choice and most importantly depends on the horse.
2. The center of balance/gravity thing you were trying to tell us about...well, again that may be your personal opinion and experience. I've seen skinny people with awful balance/good balance and the same for heavier riders. Physics only considers various constant forces and it simply can't be applied to the human body because there are too many possible variables...some people can compensate via muscle tone or having an excellent body awareness and sense of balance, etc. etc. etc.. I myself am somewhat thick and tall but I also lead an active life and don't consider my balance or center of gravity to be top heavy at all. In fact, I probably have better balance than most people....so much so that I have become an avid surfer....I promise I will let you know when my fat becomes a hindrance :P
You are making such wide generalizations about horse and people body types...that is where I think most people are having a problem with your comments. I am sure no one here would ever want to risk injuring a horse due to overloading and we all agree with you to some extent.
In any case, it is never good to hear about a fellow horse person who has been injured.