You are literally describing the breeding grounds of radicalizing young men on the internet. Microcommunities, if anything, have proven themselves incredibly dangerous to human life.
Only recent, after settling down, feudalism, growing cities, and the last blip —globalization—, moved us towards centralization.
If anything, humans are great at managing small communities, and very bad at operating in a large, centralized, globalized world. Radicalism is often a product of that globalization, and would be unable to exist in a hunter-gatherer tribal system of small moving groups.
The size of the community is probably less relevant that how it is grown. Self-selection (like e.g. hackernews) is certainly much more vulnerable to groupthink.