I don't think they are. I think they're just hardier. I can't stand watching full sized adults riding ponies (or worse, minis).
And just because ponies can carry a lot of weight doesn't mean they should. Why do you think so many of them founder? Because they gain weight easily, and carrying around all that extra weight (which pretty much equals the weight of a small rider) damages their feet. So no, I do not believe it is safe for them to carry oversize riders any more than it is for horses.
I don't even like seeing adults riding mini donkeys, and they're supposed to be the hardiest equine you can get. But they're so docile and good-natured that they wouldn't put up a fight even if they were in discomfort.
Strongest horse in the world
Does anyone else feel really bad for that pony?
This one is even worse. Take a gander at those hooves.
I hate people.