The analogy drawn between economic power and political or military power is a persistent example of a popular fallacy. People talk of the Group of Seven or G8 or G20 as the great economic powers, and they discuss the balance between them.
That language draws directly on the rhetoric used by statesmen and historians to describe the politics of the 19th century. Such language leads to talk of how China might “overtake” the US as the world's leading economy, and how the European Union might enable Europe to stand up to the economic strength of the US and Asia. Many people in both the developed and developing worlds believe that trade between north and south is necessarily unfair because of the inequality of economic power.
Insofar as such statements have any meaning, the people who make them seem to be comparing implicitly or explicitly the aggregate gross domestic product of countries or blocs. GDP is what makes military expenditure possible, but such people are talking about economic advantage rather than military force and that raises quite different issues.