In the past they were used for pulling carts and the true Dartmoor would have been multi purpose being used for riding, sheparding in some cases, as well as in a cart. I'd not want to put much more than 7 stone on a 10hh to be ccarried for any distance but weight isn't the issue really it's the rider ability. You could have a 4 stone rider bumping and bashing about causing far more problem than a 7 stone well balanced and 'light' riding ability person.
Ponies are toughies and when fit, that's the crux, able to take small adults.
The standard shetland can cope with heavier weights but it does need to be an adult not a youngster and don't forget that mature is minimum 4 years and for many as late as 6 years old.