If you have a well balanced cart there is actually very little weight put onto the horses back.. Depending on the weight of the driver, 10-20 pounds is the usual numbers you will see. Horses that drive are suppose to drive with their hind legs being where all their power comes from. They should "drive" or "propel" themselves with their back legs, They should really tuck themselves up in behind to get the power to pull. They should not pull with their front legs, for some horses it is hard to teach this to because it is in their nature to get power from the front because they are used to being ridden. It is actually less work for them to drive than it is to be ridden once the horse is use to driving and is trained correctly.
The next time you have access to a cart, have someone sit in it and then pick up the shafts, that is all the weight that is applied to the horses back when pulling. I used to have conversion charts for different weights but now I can not find them anywhere for some reason, if I do I will post them here.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"