What's the future for those Chinese Megacities? I'm not sure we see a linear/whatever development of them over the next decades. Already huge cities like Beijing or Delhi (India) are having huge problems (waste, energy, water, air quality, and all kinds of pollution, transport, ...) which easily could lead to very nasty scenarios.
The medium sized large cities (1-3 million people) usually offer better quality of life, especially if there is enough space around them.
If we look at Germany, there are competing concepts like the "Metropolitan Region" (the region with a big city or a city landscape as its center) and the "Region" (sometimes the Bundesland/State, sometimes a bit smaller). The region is defined by cultural/political/economic aspects - it could lack a big city, but have a cultural identity, which provides the bonding. The large cities were growing, not because of foreign migrants, but because they provided the better infrastructure and cultural life + the young east germans were looking for work after the fall of the 'Wall'.
At the same time in western Germany quite a bit of the economy has been driven by medium sized companies sometimes in remote/rural areas in small towns. Some of them are no longer medium sized, but kept the spirit of one. They show that it is possible to deliver high-tech to a global customer-base from small cities. Just take 'Herzogenaurach' in southern Germany, a small town with just over 20k people. It hosts three large companies: Adidas (> 10bn Euro revenue), Puma (3bn Euro revenue) and Schaeffler (>10bn Euro revenue).