There are two main concerns when a horse has to carry a heavy rider. One is the overall weight, of course. The other concern is how much AREA is distributing the weight.
A horse can often carry about 20% of his own weight. For a 1000 pound horse, 20% would be 200 pounds, rider AND tack. A very fit horse or a stout horse may be able to carry more; some breeds were specifically bred to carry heavier weights, such as the Arabian, Quarter Horse, or Icelandic pony. If a horse is made to carry too much weight, you may cause damage to his bones or skeletal structure; extra force as he moves may also cause damage to his tendons and ligaments.
If you are asking a horse to carry a heavy weight, then it is important to make sure that he can carry the overall weight; it is also important to make sure that the weight is well distributed over his back.
Pressure is Weight divided by Area (Pressure=Weight/Area). The more area carries the weight, the less the pressure will be at any individual spot (of course, the saddle must fit, and have no pressure points). Too much pressure on the surface of the skin can cause rubbing, chafing, or can slow the circulation to the muscles under the skin. Slowing the circulation to the muscles can cause oxygen deprivation in the muscle, which leads to muscle damage and muscle wastage.
Studies are not yet definitive, but it appears that a safe maximum pressure is 2 pounds per square inch. If a rider and saddle together weigh 150 pounds, then the surface of the saddle, where the panels or the bars touch the horse, should be at least 75 square inches.