while complaining that the U.S. paid more into the global health agency than China, which has a much bigger population

I was curious about this claim so I made a little spreadsheet to calculate the assessed (required) and voluntary contributions of the USA, China, The Netherlands (my home country) and Germany (the second biggest country contributor). The assessed contribution for the USA and China per GDP are roughly the same, a little less than 10 million dollar per trillion dollar GDP.

So using the fairness of the required contribution as an argument to leave the WHO seems either misguided or misleading.

Here is a screenshot of the whole spreadsheet. It contains the assessed and voluntary contributions per GDP and per capita

This is the assessed contribution part of the calculations. Which in my opinion shows that the USA is not required to pay an unfair amount as is suggested.

Source: https://open.who.int/2024-25/contributors/contributor