Given the US perspective of polarized politics, how does it make sense that the strongly-held beliefs of one portion of the voting citizens should be foisted upon the remainder of the entire population?
Smaller government is more responsive to the local conditions and culture than is large government. And the inevitable abuses of the state against the citizenry will be limited in scope.
So I think the original question is quite valid. There's a sweet spot somewhere between "as big as possible" (e.g. US or Russia) and "as small as possible" (e.g. city-states).
My gut would be that it's more determined by economic production and ethnic subgroups than some constant. Sufficient economic production to play competitively on the world stage, while grouping as many like or agreeing ethnic groups as possible (and preventing their repressing minority groups).
Or in other words, international jerrymandering.
Of course, along with very small government, I would also advocate 100% free trade and very low regulatory burden, such that a small land-area country could still be an economic powerhouse.