I do not have scientific data about this, but shouldn't smaller territories and city-states (Singapore, Hongkong etc) be easier to govern compared to actual big countries?
I was referring to geographical size. And as I said, while I do not have scientific evidence, I have a hunch that managing same size of population over a vastly smaller area should be easier.
Note that there is a spectacularly heterogenous country at the top of the same list.
https://en.wikipedia.org/wiki/Singapore#Demographics