While Chainmail was the inspiration for D&D, D&D combat was heavily influenced by a naval game. The bigger the ship, the more hit points and armor it had.As I’ve said before, some odd decisions went into D&D's design: hit points go up dramatically with level (and without magic), to-hit goes up, damage doesn't really, AC doesn't really, etc.
I would expect Sir Lancelot to last 20 times as long in combat as his squire — it's not a problem that he's that good a fighter — but I wouldn't implement such fighting prowess by giving Lancelot 20 hit dice (or nine) but no extra AC or damage.