[wisty]

Well, I think difference between China and Europe is geography, and now a common language. Mandarin is spoken basically everywhere, except Hong Kong. Even the border provinces like Tibet (Xizhan) and Xinjiang (formerly a Turkic country) have a lot of Han migrants (or colonists, as some might say). It's easier to hold a country together when you share a common language. There are local dialects and languages, but school kids learn Mandarin, and the writing is all the same (though obviously the grammar, phrasing, and idioms will be different).

As for geography, China is very flat. It was flat 5000 years ago, and peasants (the occasional ruler with a civil engineering fetish) have been making it flatter for millennia. There's nowhere for an army to hide. Dissenters can hide in the concrete jungle, but you can't make any real strongholds. Also anyone who holds the Yellow River and Yangtze River will have strategic control of most of China, and the two rivers have been linked since (IIRC) the Qin dynasty.

As for the "small towns" being a little, well, dead, that's partly a result of rural poverty (and taxes redistributing wealth to the capitals); but also (as you said) of the bigger cities being of interest to young ambitious people.

[gcb]

"to hold a country" is the key phrase there

[wisty]

Exactly. Why do you think China insists on celebrating diversity (the 55 official ethnic minorities get lots of airtime in cultural events - I've seen a national day celebration on TV in which every official minority had a representative dance troup), while emphasizing some kind of fundamental unity of the country (at the end of the celebration, all the dance troupes danced together).