The Average Medieval Knight wasn't Celtic, but on the continent [...]
Not to belabor the point, but the continent would largely either be Celtic, or Germanic (Teutonic)... Europe herself is largely one or the other in descent. And German stock is no less likely to be large in frame.
I make mention of it because of the dichotomy between histories as written by Darwinists and the actual gear on the ground. It takes a pretty big man to swing a broadsword, a battleaxe, or a warhammer, while wearing 80+ pounds of chain and steel... And ride the modified plow-horses that became warhorses - Quite often left as studs to keep the fire in them.
I understand what science portrays, and I realize my point of view may be subjective, but in my experience, individual accounts, both historical and archaeological, paint a different picture.
And while I would accept an average soldier as being stunted by malnutrition, knights were of the nobility, and their direct men-at-arms were very well fed and given, every grace nobility could afford. I dare say they were not stunted at all, and probably ate better than you and i.