Chart from the recently released White House report Opportunity Costs of Socialism:
The Nordics in their own countries (except Norwegians) are about 15% poorer than the American average. The American Nordics are 25% richer than the American average, as are oil-rich Norwegians.
The Nordics in the US are almost certainly a more elite group than the ones who remained at home – you needed a minimal level of income to afford the trans-Atlantic passage, and there would have been further selection effects for adventurousness and entrepreneurialism. The US Nordics would also benefit from the economies of scale enabled by residence in a 320 million continental empire with extreme labor mobility. Still, I don’t think that’s sufficient to explain such a huge difference.