Thats not an entirely fair comparison because GDP/capita is different, and the base assumption would be that millionaires/capita increases with GDP.

That assumption appears to hold in general (Luxembourg and Switzerland have higher GDP and significantly higher millionaire percentages than the US), but there are a LOT of exceptions, like Ireland/Norway (way less millionaires than you would expect from GDP).

This is very interesting, I would not have expected to see such significant differences between countries...