>more USians move to my "poor" country than the other way around.

Retirees and people who already made money to outbid the locals on housing and purchasing power.

Why would they move to a more expensive country where they'd feel poor?

That well-paid Americans like to WFH on Malaga beaches is not a testament to the Spanish economy, but to its weather and (more importantly) low cost of living.

In any case, don't be too sure about your claim. According to <https://ourworldindata.org/where-do-migrants-live-and-where-...>, 170K Spanish-born live in the US. By contrast, 69K US-born live in Spain <https://ourworldindata.org/where-do-migrants-live-and-where-...>. Given that the US (341 million) has 7.25 times Spain's population (47 million), this means that a Spaniard is 17-18 times more likely to move to the US, than an American is to move to Spain.

If we were to take all preceding arguments seriously, couldn't that be explained by americans going to live in a low-cost-of-living country once they've earned their living?

(Don't know your country to know if that's true or not, but it's a possibility)

The demonym is American, not whatever you wrote. Unless your poor country is New Zealand, that's not the case.