If I remember rightly @Jessey - and again I'm talking from memory so could be wrong - then that study was later found to be flawed. It was a small sample, the horses carrying heavier riders weren't all used to that, and the same tack was used for each rider on the horse so while it may have fitted the horse it wouldn't always have been suitable for the rider and would have put them off balance and/or in a poor position. I'm not saying weight isn't an issue, but there is far more to consider and a lot of faith was put in a flawed study. I know I've said it before but Little Un would happily carry me and look sound, yet the day we put my friend's I think 5 yo daughter on him who was very light he quickly looked lame because she wasn't balanced enough on him and he wasn't used to an unbalanced rider. I doubt she'd have been 10% of his weight including tack!