But if current dominant country A has GDP of 100 units and rival country B is poised to get to 150 units, then won't country B be able to wield more economic power than country A even though country B's GDP/person is lower?

It's sort of like a company that has 100 units of revenue and 20 units of costs versus a company with 150 units of revenue and 90 units of cost, the one with greater revenue may still have more economic power (e.g. ability to leverage suppliers, etc) despite having lower profit margin.