Just assuming a normal curve - regardless of the threshold of "minimum ability to count as a researcher" one would expect between two comparable populations of different sizds more capable of and actively driving research as population grows.

Higher GDP is correlated implies higher per person productivity and more specialization to decommoditize themselves vs the global labor. Plus pushing the envelope often requires more funding for novel projects as there isn't an off the shelf for something novel to reduce costs.

They aren't perfect measures by any means (there exist things which boost either not research) but are decent enough proxies.

A single researcher is rarely productive on their own. Effective research requires a tight network of likeminded individuals and support structure.

If the number of corporations needing new discoveries keeps growing but the funding is fixed, that means either the researchers are spread thin or there’s a centralization of research that has a hard time transitioning into the wild. (Both situations exist, and it sucks.)

Replace 'single researcher' with 'a fixed size research group' in my comment.

Your argument about a minimum size research group for productivity is interesting. I am not quite sure whether that means we should normalise by population size, though.