History-ironically, I (as an American) would be less worried if Germany were rising at China's pace.
Maybe it has something to do with population size, but IMO it is more to do with their economic structure and system of government. Japan is a democracy, and they believe in markets. Since War 2 they also seem a little less pushy with their neighbours...
To the extent that Japan was encouraged and did "voluntarily" limit migration.
How the wheel of time turns... different players, same games.
Where do you get that idea? Germany is one of the most active countries in diplomacy and is at odds with the US all the time.