This is what Kris said about it:
I've heard that rider balance is more important than rider weight, but I wanted to PROVE it. So yesterday I completed an experiment. I rented an Infrared Thermographic camera and had two very different sized riders ride the same horse in the same saddles at the same gait for the same amount of time. There was 100 pounds of difference between the riders. The heavy rider rode relaxed and in balance, and the lighter rider intentionally rode stiff, crooked, and out of balance.
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