Oh what a bucket of worms.
Let me start out by saying I'm sorry if I repeat something that's already been said, but I'm not going to go through reading 13 pages of comments
. Also that there really isn't enough time or space to cover it all here.
I always love it when someone asks me if horse "A" can carry person "B" and they're using the persons height and weight compared to the horses height and weight. One of the things I always want to say (and have said, but only to someone I know really well and won't get upset with me) is - do you want to know if the horse can carry it or should carry it?
It's one of those cases where a little knowledge is dangerous.
There's a LOT more to it than just the weight ratio.
If you really want to look at the weight a horse should carry you can't just look at the weight ratio of rider and tack to horse. You also need to look at the size (circumference) and density of the cannon bones as well as the size of the horses loins. For example a horse usually has cannon bones that are the right size for carrying it's own weight, but when you put the saddle and rider on it's no long the size it really should be. There is a formula for it and I'm sure if you google for it you should come across it. Ok, I did it for you http://www.horseprotection.org/id51.html
. Not a bad article. Covers much of what I was going to say (or already have). So you can read the article and I can move on.
All that being said it doesnt mean a heavy person can ride. You just need to take more than just the weight ratio into account. However for just weight the 20% of less rule is best providing you look at the total picture.
I'll never be able to over state the importance of a well fitting saddle unless you're just going to walk around a ring for 15 min a few times a week.
You want the most surface contact from the saddle tree (i.e. The bars). The tree displaces the weight, so with a well fitted saddle the more tree you have the more weight you can displace. That's why the over 160 they came out with the early version of the Universal Pattern military saddle to replace the Hungarien/German saddle (in the US we call it the "English" sddle) that had be developed for the Cav, but had a bad history of leaving too many horses unfit for duty during long campaigns (doesn't displace enough weight over a large enough area). Anyway, you really should have a well fitting saddle.
Riders ability matters too. Balance, balance, balance. I won't get into it all. If you can't ride it's not going to be fun for the horse no matter how light or heavy you are (it will be just more unpleasant if you're heavy).
No I have to ask. A 15,1 hand QH that weighs 1100 lbs?
My almost 16.3 Georgian Grande is 1214 lbs. My 16.1 hand GG is 1317 lbs and I'm working her more in an effort to take off at least 100 lbs.
Please watch the weight of your horse. But that's a different topic (although an equally important one since weight can lead to so many health issues for a horse).
And although it off the topic I'll post a picture of both these horse together and you'll see how much better the younger/taller one looks at the correct weight. While 100 lbs might not seem like a lot when talking about 1200 and 1300 lbs horses it's weight that she doesn't need and shouldn't have. She'll be healthier without it.