I find that "size chart" a bunch of hooey. In my experience, anything that's trying to give you a mathmatical equation to determine things like height and weight are just bogus because they do not take into consideration the outside factors.
For example, the "20% of body weight" rule. How on earth does the horses weight determine how much it can carry? For both this charts, you would end up with a very small person riding an Arabian for example (don't weigh much, and impossibly tiny cannon bones). And yet Arabians have been bred and equipped to carry MUCH larger people based on their bone structure.
Likewise, look at a Draft. Based on both charts, they weigh a lot and have enormous cannon bones so they can carry the heaviest people right? WRONG. Drafts are actually notorious for having a much weaker bone structure due to the immense weight they already carry on their frames and being bred to pull, not ride. A very large man or woman in the 300+ pound range is actually better suited to riding a thick sturdy QH or Mustang type animal then using a Draft!
What about ponies? Based on those charts, they should only be carrying very small children based on weight and cannon bones, and yet they are often easily able to carry riders just as big as QH's and Arabs can manage.
It all comes down to a better knowledge of specific breed bone structure then trying to use silly equations to determine things. According to a standard BMI chart, I am considered obese. I'm packing a few extra pounds, but you can just bite me BMI, because obese is a lark. It takes zero into consideration about my bone structure and my health. I am overweight for sure, but obese is ridiculous as far as I'm concerned.
That being said I am 5'6" and I weight 185lbs. I ride a 14.3hh Arabian who has never had a single issue carrying me in the almost 11 years I've been riding her. I have ridden ponies as small as 13hh who never had an issue. So in the end, unless you're massively obese, the vast majority of horses and ponies will NOT develop problems from carrying you.