(Is "Are big societies optimal for human happiness/quality of life," a fair rephrasing of your question?)

That's again a pretty trival answer. No society isn't optimal. We don't live in utopia.

Since both humans and societies got to where they are through mostly unthinking processes

Nation states are created via human made law and a lot of the ways humans interact with each other socially in modern society got thought up by humans as well.

Without wanting to get into bad evolutionary sciences, I think it's reasonably fair that even modern humans are mostly adapted for the hunter-gatherer life, with a couple of more modern modules thrown in. It's also reasonably fair that humans were mostly "made" to live in small tribes, hunting and gathering.

What does that mean? That modern humans have a lower lifespan than they would have in a hunter-gatherer life? That happiness is higher?