not the OP, but this was covered a fair bit in a book I read recently called "genghis khan and the making of the modern world" which was a pretty good read.

Iirc it was that the figting forces of the various northern chinese states were comprised of conscript peasants whose diet consisted of grain and not much else, which stunted their growth among other things, meaning the mongols were significantly larger and stronger than them leading to certain mythologizing in the centuries afterwards. It probably also didn't help that the conscripted peasants were, well, conscript foot soldiers, wheras the mongols were willing participants on horseback

Thanks!

Bang on, same thing with the Dutch and the Brits when the empire flourished.

Dietary changes drive a lot of these things, more than I think is generally understood and credited.

The Mongols are a very famous example and extreme as you stated the Chinese peasant and soliders mostly had a grain based diet and poor health that came with it.