Got it!

You know, it also occurs to me that it could ALSO be a function of the fractured nature of Europe at the time (and now).

Really many of the inventions we are talking about just require a single reasonably financed, reasonably smart, reasonably motivated individual to spend a decade or two and drive some innovation step forward.

Because china's intelligensia is more centrally organized we dont have as many rolls of the dice. In Europe each little city state gets to roll the dice to see if it has a smart, motivated, finance guy to push one innovation step forward. Each city state is in a unique situation so there are more chances for more directions of exploration.

In china they would have more folks thinking and acting in unison, but there would be many fewer independent contexts for this to happen inside of.

If this reasoning is correct, then there is probably a magic ideal size for each age. I suspect the ideal size has gotten larger. Now it really takes an army to make bigger gains. e.g. only the biggest concentrations can push the limits of lithography, quantum computing, AI etc.

Of course the present world is also pretty incomparable, since now the internet has really caused all of humanity to really operate much much more closely, so no group is a separate as they all used to be.