People also like to forget that these economies expand by attracting people. A city is productive because it has people and it attracts people by being productive. Cities still require the resources of the regions they're in (those resource include all land labour capital and entrepreneurial). The US and its major internal economies benefit the same way from being part of a global economy. These major US economies are little more than cities on a global economic scale.

How much of Californias economy would exist without domestic or international migration? How much of that migration occurred because it is a big economy?

What might be interesting to see is gdp by generation of arrival.