In the riding school we had a Welsh Mountain pony, Bernie, 12.1
One of the owners bought him off a Welsh farmer one winter after seeing the farmer. About 170 lbs. riding him up the sole of a mountain in two feet of snow. The farmer had a bale of hay strapped to his back amd another across the pony's neck to go feed the sheep.
Some of the native ponies, the Fells and Dales, Welsh cobs, Highlands are not any bigger than 14.2 buy capable of carrying a man for a day's Foxhunting.
I at (then) 140 lbs have hunted a 13 hand pony all day, I have more than once got on very small ponies to teach them manners. Shetland amd Welsh ponies pulled coal carts up through the mines, weights exceeding theirs by a long way.
One Exmoor pony liveried at the riding school, was a real PITA with her small owner and refusing to go away from the stables. It was reported that she was tearing right up and bucking. I had a right royal argument with the little toe rag and was still fighting her past a large old house that was being converted. The builders started calling out, "Cruelty, get off and carry it!" As well as many other unprintable things.
The mare decided they were on her side and started to stand right up. At this point one of the men said "Hey, that's the little pony that was so nasty to the girl yesterday!"
Their tone change to "Ride him cowboy!" and telling me what to do to her!
I rode that pony for three miles along the beach, most at a canter, amd then trotted her back. Counting the times I turns her about to go back away from home she probably went ten miles at least. She wasn't at all bothered by the extra weight, only that she couldn't get her head down and her butt up!