Here's what I found on weight...
When asking the question, "how much weight can a horse carry?" you often hear a response similar to "it depends on the breed of the horse, its conditioning and conformation, how far and how long you will be traveling, the horse's bone structure, the type of weight the horse will carry (for example live [rider] or dead weight [gear]), weight distribution" and so on. So with this sound advice from your fellow horsemen, surely you now have a much better idea of how much weight your horse can carry. No? Of course not. Though your colleagues are correct in stating all the above factors, you are still left in the dark without any guideline on how much weight a horse can carry. Can my horse only carry 80 pounds, or is he capable of carrying 300 pounds?
Fortunately some very smart horsemen over the years have come up with a few methods for us to calculate a starting point to help determine how much weight a horse can carry. One very simple guide is to take the horse's weight and divide by six to give you the total weight, including rider and tack, the horse can carry. Given this, a 1200 pound horse could carry up to 200 pounds.
Another quick and popular method is to use the 20% rule. You take 20% of your horse's body weight and the result is the amount of total weight your horse can carry. For example, a 1,000 pound horse should easily carry 200 pounds of rider and tack. The 20% rule typically applies to competitive or otherwise "hard" riding. For pleasure riding, many use a 30% rule, so this 1,000 pound horse could carry 300 pounds for shorter pleasure rides.
Measuring a horse's cannon bone is used by some in determining the approximate weight a horse can carry. 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. Using this method, I measure the circumference of the horse's cannon bone and get 7.5 inches. The body weight of the horse is 1,150 pounds and the rider and gear weigh 235 pounds for a total combined weight of 1,350 pounds. Divide 1,350 (total combined weight) by 7.5 (cannon bone) and I get roughly 185. Divide 185 in half and my final resulting number is 93. Using this calculating method, I either need to lighten my gear or get a larger horse to get the number down to around 85.
Some horse and rider guidelines where carrying weight is concerned:
Pick a horse with bigger cannon bones, wider loins, shorter back
Avoid using heaving saddles and only carry necessary gear
Make sure the horse is conditioned for the type of riding you doing
Keep proper riding posture and balance
Give the horse a break on longer rides - get off a while and let your horse rest
Avoid riding in areas where footing is not desirable - such as mud, deep sand, asphalt
Avoid letting the horse trot or canter
The maximum weight a horse can safely carry does vary by the breed of the horse and how hard it's worked. There is no absolute rule about how much weight a horse can carry, but generally speaking the lighter-framed the horse the less he can carry. A well conditioned horse or a stout horse can generally carry more. Some breeds are bred to carry heavier weights like the Quarter Horse, Arabian or Icelandic Pony. Riders with good balance also make weight load less of a problem.