> Why specifically GDP? [...] Why not PPP, for example? PPP is effectively a currency conversion rate, it's not a meaningful metric by itself anymore than $1US

> Why specifically GDP? [...] Why not PPP, for example? PPP is effectively a currency conversion rate, it's not a meaningful metric by itself anymore than $1USD ~= 1000won or ~= 100 yen tells you anything about the South Korean or US or Japanese economies. So I'm assuming you're comparing nominal GDP versus GDP by PPP. To a first approximation, nominal conversion rates give you an idea of how an economy can interact with others on a trade basis, whereas PPP conversion rates give you an idea from the perspective of a domestic consumer. For what you tend to use total economic size for, nominal GDP tends to be a better reflection of the size of the economy, whereas PPP works better for something like GDP per capita.