The 20% rule was established as best for the horse. The reality of what things weighed don't always cooperate. And the Army has always done things it's own way.
Medical experts told the Army for years how to reduce shin spllints, but it wasn't until the late 1980's that they started being listened to.
Around 1900 the prefered weight of a US Cav trooper was under 150lbs, probably averaging about 140 (they watched weight for troopers then just like they do height of pilots now....Maximum allowed weight was 165, but over 150 was suppose to be the exception...probably done for the sake of some officers :)) ). Saddle, carbine, etc... (equipement) came out to 90 lbs. Average Cav mount in 1900 was just over 1,000lbs. (Morgans and Saddlebreds were popular probably for their size, endurance and gait, but any horse that measured up could serve)
That would put the average weight carried just under 24%. And while no doubt some went over 25%, more were likely to be closer to 22%. Not a bad ratio when taking into account that you're dealing with the military.
European nations had different requirements and made fewer long range demands on their Cav, so weight was less of an issue
If you can find it, "Horse, Saddles, and Bridles", by Col W. Carter is an informative read. He wrote it around 1900, so it might not be to easy to come across, but Amazon
might have it.
As for carrying 20% (or 25%) of our weight. We don't to have test our endurance by traveling 300 miles in 5 days carrying over 20% of our weight (a qualifying test for Cav horses). Somewhere I'd read about an even more demanding one of 100 miles in one day caring about 30%, but I'm not so sure about that one.
As for the 170lb + ground pounders of today. We are larger then we were over 100 years ago. Well, some are. I was 147lb at my ETS physical. Gee, I just could make the qualifying size for a horse solder, but they didn't have the horse Cav then (I'm not that old). :)) Very accurate comparison between humans and horses with regard to effect of carrying loads. Riding a horses is actually not the most effecient means of travel (taking into account the feed, care, etc...). Better off having it pull a wagon. Slower, but more effecient. But for me riding is a lot more fun than walking the whole way. And wagons are more restrictive.
Still, today I can find saddles of the same weight and lighter gear (I don't need the carbine and ammo), so at 160-165lb + saddle and equipment I can keep it to 20% (especially with my 1300lb mare :)) ) Obviously a healthy horse, with no more demands than most of todays riders make could probably manage 30%, but I prefer not to push it. Not many people ride 60 miles every day for 5 days (I know I've never done 60 in one day....50, 30 years ago, but never 60).
Very glad to see that there is someone else out there who takes the time and intrest to looking into the weight we ask our mounts to carry. I never really worry about it for one day rides of 20 miles or less. I think any healthy, conditioned horse with a proper fitting saddle can probably carry at least 30% with no problem. For me weight only becomes an issue with long distances that require many days. As I point out to my children. For long distance, week long rides it's not distance that will do the horse in, but the weight you make it carry. (So we won't take the caste iron pots on those rides) :))