I like looking at purchase power in general, rather than GDP - the absolute numbers don't matter so much as long as your domestic market is large enough to produce most things people need.
Secondly, the density of savings and taxes per square kilometer is giving you an idea about how much potential for development a current settlement has. That's the capital that can be drawn from directly. Density of spendings (train tickets, consumer goods etc.) is the driver for outside capital to come in trying to get a dividend. Taken all together (savings + spendings) you get the density of PPP adjusted GDP per square kilometer as a figure that matters. As you can see in [1], US and China is actually quite similar in this metric, and it's probably also still similar if you just take its coastal regions. I also think that this metric is very similar for the population belt between London and Rome, and the East Coast corridor in the US.
I like looking at purchase power in general, rather than GDP - the absolute numbers don't matter so much as long as your domestic market is large enough to produce most things people need.
Secondly, the density of savings and taxes per square kilometer is giving you an idea about how much potential for development a current settlement has. That's the capital that can be drawn from directly. Density of spendings (train tickets, consumer goods etc.) is the driver for outside capital to come in trying to get a dividend. Taken all together (savings + spendings) you get the density of PPP adjusted GDP per square kilometer as a figure that matters. As you can see in [1], US and China is actually quite similar in this metric, and it's probably also still similar if you just take its coastal regions. I also think that this metric is very similar for the population belt between London and Rome, and the East Coast corridor in the US.
[1] http://mecometer.com/topic/gdp-ppp-per-square-kilometer/