Ouch, The average light-bred horse is about 1100 pounds (500kg) and it is suggested that they should not be asked to carry over 20% of their own weight.  So a fully loaded light-bred horse should not be carrying more than about 220 lbs (100kg), considering the tack and supplies are about 40 pounds or so (with some working saddles weighing 60 pounds or more)... needless to say, these ladies are stressing the shit out of those horses. Now, you can train a light horse like you train a human.  Ask it to carry a significant amount of weight for a good amount of time over and over again and the horse will become accustomed to doing it.  But to ask a recreation horse to carry a metric shit tonne of weight when it normally doesn't with cause discomfort and muscle pain to the horse. http://www.equinews.com/article/horses-weight-carrying-ability-studied