I think you’re probably right there, Daed. I doubted my own findings so I consulted a few horsey people (breeders, riders etc) A horse can comfortably carry a third of it’s own weight. You’re average destrier of 15 hands weighed 1200 lbs. So they could comfortably carry 360lbs.
Assuming the average height and weight of a knight was 5’7″ and 150lbs (tended to be noble so ate meat often growing up and were therefore taller than peasants) and the average weight of a set of custom made full plate jousting armour (which was thicker and heavier than regular armour) was 55-60lbs. So the destrier was only carrying around 200lbs plus whatever weight the weapons used came to. Not even a third of its weight. Plus many knights in 1300-1400s inherited armour which was ruinously expensive. Most could not afford custom kit so they mixed and matched mail with pieces of plate for ares like the chest, elbows, knees, feet, shoulders and hands. This mix and match kit was significantly lighter. So you’re right, I think people assumed that armour was way heavier than it was and consequently you needed a 17 hand draught horse to bear the knight!
I have given this way too much thought 😂