You occasionally hear people say this, but what reasoning is behind it? Have there ever been any studies done that show ponies are anything other than little horses? I just can't see why they would be innately stronger, more able to carry weight than their larger counterparts. I imagine, just like big horses, it's more about their build, maturity, fitness, etc, etc as to how much they might carry healthily. Therefore the (*average of*) 20% guideline should no less apply.
Let's look at it from a physics POV, and add in some kinesiology.
This isn't true of all ponies, but it has to do with bone, shortness of back, and overall conformation. Ponies just tend to come in a "design plan" that lends itself well to carrying weight.
Typically, shorter horses with thicker bone and slightly coarser builds tend to be better weight carriers that something that is taller, but more finely built, with a longer back.
My favorite example of this concept are the Icelandic horses and native British ponies. Most Icelandics don't weigh much more than 800-900 lbs and rarely get over 14 hands, but the fact that they have thick bone, wide frames, and short-coupled backs means they don't have much trouble carrying people of a larger stature - most Icelandic horse rides for tourists top out at 225-250 lbs.
Think of it this way - if I'm carrying a heavy backpack, and choose to hold it out in front of me with my arms straight rather than on my back, that's going to tire me out a lot faster than carrying it close to my center of gravity, on my back.
If I have a load closer to my center of gravity, I can also use the bigger muscle groups in my back, hips, and legs to create the force I need to balance, since I'm now placing that load on what amounts to a pillar
instead of a string.
A pony has a closer center of gravity from each end of its body than an OTTB would. Therefore, it doesn't also need to lift and balance it's neck, back, and hindquarters in as big of a movement as something with a long, finer frame would, so with that too, you're also limiting the amount their muscles have to work to constantly re-adjust both their bodies and the load to stay in balance.
Basically, the force the pony has to exert might not be as great because it just doesn't need to make as many adjustments to stay in balance.
Can the TB become conditioned to be a weight carrier, and could the pony be a poor one if it's not conditioned right? Absolutely. But it would take a lot of work on the TB's part to condition the right muscle groups, and he'd still probably not have as much stamina as the pony, since his muscles will still need to make a lot more adjustments.