I don't see any problem with plus size riders so long as their mount are suited. While 20% is a good starting point, I do not believe in it. For one, I have seen a 13hh haflinger cart me around better then a 16hh+ warmblood. The 20% rule doesn't account for tack fit, riders ability (or lack there of), type of work, terrain, length of the ride, horses conformation... I'm sure there's about a dozen other variables! Do all horses have limits, absolutely, but I think each horse should be evaluated on a case by case basis.
That said, I'm working on losing weight and having GREAT success (you can see progress photos in my Ketogenic thread). Losing 45lbs has greatly improved my overall riding! My balance is better, I have more stamina, and I just feel so much better!