using GDP alone biases towards countries with large populations (like China) which might not have the level of resources implied by a high GDP as their large population drains much of that wealth.
On the other hand, using GDP per capita biases towards very small rich countries, that may not have the power implied by their high GDP/cap, as their size limits how powerful they can be (e.g. Singapore).

So, you instead use a measure in between the two, and the naive way of doing this is by multiplying the two measures... (read more)