One piece of evidence for the second is to notice how nations with small populations tend to cluster near the top of lists of countries by per-capita-GDP.

That's probably mostly a statistical artifact of small nations being more numerous and having greater variance in conditions rather than a significant governance difference. There are also many non-statistical confounding factors, such as nations which allow/promote immigration (slightly) tending to more "average" GDP per capitas, and higher populations; small nations having more homogeneous populations which increases social stability (and which may not be possible to replicate by a breakup of large nations); some small nations perhaps undergoing a sort of "gentrification", wherein poorer inhabitants choose to leave due to high prices, while wealthier ones move there (not as sure this one is valid).

Overall still downvoted for general agreement.