I believe that you should not put more then 15% of the horses body weight on their back no matter how tall the person or horse is.
It greatly depends on the horse and their conformation. Some horses are better suited to carry weight than others.
Just as an example, both these horses would likely weigh somewhere around 900 pounds
This horse is probably taller, somewhere around 15 hands, but is much less suited to carry weight because of the long, weak back
Whereas this horse, with the short, strong back and large, well sprung barrel could easily carry a lot more weight than the first horse without issue...
There is a lot more to deciding how much a horse can carry rather than just looking at their weight/height. It all depends on their conformation and whether they are suited
for weight carrying.
That second horse above, only being 14.3 and weighing maybe 900 pounds, she could carry more weight than some 16hh+ 1400 pound draft crosses I've seen.
ETA: Back to the OP, yes, I'd say you are too big for the pony. Is there maybe another smaller rider at your barn or nearby that would be willing to help with the training?