It depends on the horse and the rider (and saddle fit!).
On the horse you need to look at the strength of the loin area and the thickness of the cannon bones and the overall conformation of the horse. The height of the horse also plays a factor - taller horses have a higher center of gravity which makes it harder for them balance versus a shorter horse. Plus you have to look at the overall fitness of the horse.
Think of two men that weight the same. One is a short stocky guy that is all muscle, and other is a tall, lean-boned guy with a bit of a beer gut. They both weigh the same - but their carrying capacity will be different.
As to the rider - it depends on their balance and seat. A heavy rider who has a well-balanced and light seat is going to be much easier for a horse to carry than a light-weight rider who is all over the place.
So a stocky but fit pony is probably going to be able to comfortably carry 25-30% of their weight, while a taller and leaner TB is probably going to be closer to 15-20% of their weight. If the horse is overweight, their weight carrying capacity is going to decrease.
And keep in mind - Dale ponies (or was it Fell? I always mix those two up) were used to carry +400 lbs through the hills on a daily basis and stayed sound - and they are stocky ponies and that is dead-weight which is harder than live weight that can move with them. Not that I would recommend that by any means, but just something to keep in mind.
Here is a popular forumula that includes a cannon bone measurement and the horses own weight and the rider's + tack to determine if you are ok:
"A measurement is taken around the circumference of the foreleg, just below the knee. Add together the weight of the horse plus the rider and tack, and divide this sum by the cannon bone circumference measurement. Then divide that result by two. A number between 75 and 85 is good. If the number is over 85, you probably need a larger horse." Bad news for heavy riders and narrow horses | Horsetalk - International horse news How Much Weight Can an Icelandic Horse Carry? How Much Weight Can a Horse Carry?