I know in the past horses were too small to ride only one (hence chariots were a thing until they could breed larger horses.) I'd say a healthy noble in full armor with a weapon and whatever other gear would be near or over this limit, anyone care to explain how they worked around this? Was it simply breeding larger horses, or did knights maintain a diet or try to avoid becoming too large?
I'm afraid most of your premises are quite off, especially in terms of weight. If anyone would like to present a fresh answer, you're more than welcome to do so - I've been fighting this myth for ten years, it'd be nice to have some help. In terms of previous answers, we do have some. As this is a horsey matter, we can turn to u/PM_ME_UR_SADDLEBREDS for some answers:
u/Hergrim also examines knightly horse-riding and the relevant weights.