The four horses I've ridden enough to discuss intelligently max out at 900 lbs. My 180 lbs plus a 35 lb saddle gives the following percentages: 24, 25, 28, and 33. If Mia or Trooper have any trouble carrying me, they sure hide it well (24 & 25%). Lilly was very green broke when I rode her, but after 6-10 rides, she did fine...although I could sometimes feel her making adjustments for me when we did tight turns (28%). Cowboy can carry me, but he is adjusting his stride and particularly has to adjust in turns. At 33%, I'd say I'm too heavy for him for extended riding - but like a lot of ponies, he is structurally strong for his size. His legs are as big around as my larger horses, and he has a short back and broad loins for his height (13 hands). He shows no sign of strain when galloping with me, but we almost always use him as our third horse for trail rides - so I almost never ride him. Based on how he moves, I'd be concerned if I rode him regularly or long.
All of my horses are fairly slender. For western riders to reference, they use a 26 or 28" cinch, and I need a nylon latigo so I can loop it 3-4 times. For my Aussie-style saddle, I use a 30" girth, but I'll eventually replace it with a 28" - which is the next to smallest that DownUnder sells.
Based on that limited sample, I wouldn't worry too much at <25%. Above 25%, I'm going to want to think about it. After 30%. I'd want to watch carefully - can that specific horse, with his experience and conditioning and conformation, carry that specific rider? And WHY am I asking him to do so?
I think percentages give you a marker to say, "Whoa! Does this make sense?" Some may put that marker at 20%, which is fine.
Ponies will have higher percentages, but many ponies are built well for carrying a higher percentage. Arabians have a reputation for carrying heavy loads, but mine are both slender. I'd bet a 220-230 lb rider could sore Mia or Trooper, if they were not experienced riders using good fitting saddles. 30% for Mia would be 270 lbs (235 rider + 35 lb saddle). I'm pretty sure my tall, slender prima donna mare would protest...and I honestly wouldn't ever ask it of her. She is my biggest horse, but also seems to have the most sensitive back. She also has a strong preference for men over women, and would probably try a lot harder for a 220 lb guy than a 220 lb woman.
A lot of trail riding places set a 250 lb limit on riders, and some set it at 200 lbs. I can understand that. I am certain there are horses who can carry a 300 lb person just fine, but there are not a huge number of them.