A cursory look would imply that each color segment increments population count in some sort of consistent and logical way, but it doesn't.
The increments are like so:
0 to 1 million
1 million to 5 million (5x increase)
5 million to 10 million (2x)
10 million to 20 million (2x)
20 million to 50 million (2.5x)
50 million to 100 million (2x)
100 million to >500 million (5x+)
That last category is the most misleading, because it paints the US, Russia, China, India, and a bunch of other countries as being in the same league, when they're not even close.
The details show the US at 320 million people, Russia at 144 million, the China at 1.4 BILLION and India at 1.3 Billion.
It's insane that these are grouped together. Totally nonsensical. The difference between China and Russia is about 10x. [UPDATE: My initial bad math said it was a 77x difference.]
</rant>