LOL one of those impossible to answer correctly questions!!! (due too many if/but factors)!!
A lot will depend on the ability of the rider (unbalance riders are hard for as horse/pony to carry).
Additionally the rider proportions need to be considered - It doesn't really matter if the pony could carry the weight or not, if you can't fit a big enough saddle to distribute that weight evenly (due to the length of the ponies back), then a bigger rider isn't going to be comfortable for the pony to carry.
Then the age, fitness level, health and training level of the pony needs to be considered. Not to mention what the rider is planning to do on the pony!
I don't think animals should carry the max the whole time so about 10 stone. A heavier rider getting on for a few minutes is a different matter. PiebaldSparkle has put a lot of good points forward which definately need to be thought about.