>Using this approach, Singaporeans are more likely to move to China than Chinese to move to Singapore.

Using the Our World in Data figures, a Singapore-born person is about four times more likely to move to China than a China-born person is to move to Singapore.

That does not seem like an unreasonable ratio, given that a) Chinese are not nearly as free, whether economically or politically, to emigrate, b) those Chinese who have the money and connections to leave are likely going to be in the large cities that offer good living standards, and c) China is going to have, from sheer size, aspects that will appeal to Singaporeans regardless of per-capita economic disparity. Put another way, if Chinese were as free to move around the world as Singaporeans, there would surely be many more China-born people in Singapore ... perhaps four times as the current 530K.

By contrast, the US and Spain are fairly comparable in terms of their citizens' freedom to move about, so that factor can be eliminated from the comparison.