Bear in mind that you can interpret Aurora population numbers as employment totals rather than total population. This lets you count the 'Aurora populations' of countries lower if you so desire.
Or you can futz the populations of low gdp countries to be lower. This is very relevant in the case of China; compared to the US or the Euro countries China has enormous agricultural employment- 40% according to the world bank, compared to 2-3% for the US and 5% for the EU nations as a whole. It is not too far to say that you can safely discount 500 million people or more from China's population for Aurora wealth generation.
I would agree with Zook's suggestion on finance centers - it might be a far better way of simulating real-world financial advantage than futzing the wealth percentages.
If you apply these suggestions it might look something like this.
For example, though the China based faction has almost three times the population of the U.S. based faction, it no longer earns almost three times the wealth via taxes. Instead, it earns about 40% of what the U.S. based faction does as wealth via taxes.
base: 1.3 billion Chinese, 311 million Americans, 332 Euro-Areans.
--> 800 million, 300 million, 300 million nonfarm employment.
(Doing a faction with only 300 million people of base industry kind of sucks, by the way!
Hmmmmmm... You can fit 120-130% conventional start industry plus 300 financial centers in 300 million people. Do 50% industry for China (or less!) and you get reasonably fair starts.
Give the US and Euro a level of free Wealth Tech and it should even out in terms of wealth.
Really, unfortunately, to balance China without fiddling with wealth % (and the problems that causes down the line) you probably have to reduce the pop even further. Or the industry even further, though that still gives China a lot of wealth. I would reduce everyones wealth % to 80% or so in this scenario.
Sidenote: Chinese GDP has been radically increasing in terms of USD. In 2007 it was 3.5 trillion, now it's 7.3 trillion. So if this set up is near future at all you can feel free to futz their wealth income.