If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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The scatterplot shown here appears to show a strong positive correlation between population and GDP per capita.
[EDITED to add: no, I'm an idiot and misread the plot, which shows a clear correlation between population and total GDP and suggests rather little between population and per capita GDP. Sorry about that. The Gapminder link posted by satt also suggests very little correlation between population and per capita GDP. So the context for my (unchanged) argument below is not "Increasing returns to scale are just one factor; here are a bunch more" but "Increased returns to scale are probably negligible; here are a bunch of things that aren't".]
In any case, "increasing returns to scale" were just one example (and I think not the best) of how someone might be more productive on moving from (smaller, poorer, more corrupt, less developed) country A to (larger, richer, less corrupt, more developed) country B. Here, let me list some other specific things that might make someone more productive if they move from (say) Somalia to (say) France.
And what caused these differences between these two countries? (Hint: it's not magical corruption ray located in Mogadishu.) And how will these traits change as more people move from Somalia to France?