Id say it depends on the AGE, the physical condition, the build (long/short back, long/short cannons, light/mid/heavy weight and whether it has any skeletal deformities, such as severe cow-hocked etc) and definately HOW MUCH BONE the horse has! Also, Id take into consideration how long the horse is going to be carrying a heavy rider, what work he will be doing with a heavy rider, and also the balance on the heavy rider!
However, there is (apparently) a formular that measures the amount of bone a horse has, and times' it by something, (or inverts it, or does something with it), and gives you a guidline as to how much weight the horse is comfortable carrying. But I dont know it, lol - try looking on net!
My 16.1hh IDxTB has a good 10 inch bone, short cannons, but a fairly long back. Hes 4, but not ridden all that much - both me, and the other lady who occasionally rides are both 5'10, but are both around 10st....so he carries us no problem! He also has no problem carrying my husband who is 6'2" and around 14st...but hes only ridden him 3x....and for about 10 mins at a walk...but given my horse is well built, and has good structure, I'd say give him another 2 years, and Ill get my husband to do XC on him, lol...needless to say my horse has no quams about carrying a 3 yo and a 5 yo, lol.
Seriously though, if your horse has good solid bone, and is above 6 years old, then Id guess that it could quite happily carry 12-14st....dunno about jumping and strenuous activities though - but Im no expert! can you ask a vet/back specialist?
Can you post a pic of your horse (include legs), and tell us its age, and how much its done? people might be able to get a better idea from that!
Ashlea x