Historically, too big to govern would apply not to the number of citizens but to the time information needed to cross the empire. Hence the famous Rome roads,

Historically, too big to govern would apply not to the number of citizens but to the time information needed to cross the empire. Hence the famous Rome roads, the Nile for the Egiptians or the excellent communication system of the Incas (there are quite a few other examples) that made those empires possible. China might have been historically to big to govern with some stability, but with the high speed communication networks of the present that might have changed.