Scandinavian countries aren't >25M, neither are they "most [...] countries".

EDIT: I started thinking there's more to this argument. Scandinavian countries are more the size of the average US state, than the size of the US itself. Comparing Sweden to Michigan makes more sense to me than comparing Sweden to the US (also makes sense from a GDP perspective: Sweden had a 2012 GDP of $523 billion, while Michigan had a GDP of $408 billion; they're close in both population and wealth).

The GP's point was that the US is rich for its size, which IMHO is testament to the effectiveness of good federalization (the US being a federation of 50 semi-self-governing states). IMHO size does matter, and it's really hard to govern huge countries.