I don't think this necessarily true. A community still seems like it can stay somewhat decent in terms of quality if the the guidelines are clear, they're enforced equally, useless content isn't tolerated at all, etc. Seen a few forums do quite well in that sense, and quite a few fandom/hobby specific wikis do well too.
But the odds are severely against it. Usually the rules aren't enforced enough, or equally, either because of an obsession with metrics over community quality (read, every corporate owned community ever) or because it's simply too difficult to do. And once low quality content (and the people who post it) gain a foothold there, it's probably game over. See every webmaster forum in existence. Or almost every subreddit about the same too. Things like DigitalPoint, Web Hosting Talk, etc are garbage now, since spam and fluff posts were basically left to run rampant, driving away those who'd post thoughtful, interesting discussions.
So technically no, there are communities that stayed good with more people. But usually yes.
But the odds are severely against it. Usually the rules aren't enforced enough, or equally, either because of an obsession with metrics over community quality (read, every corporate owned community ever) or because it's simply too difficult to do. And once low quality content (and the people who post it) gain a foothold there, it's probably game over. See every webmaster forum in existence. Or almost every subreddit about the same too. Things like DigitalPoint, Web Hosting Talk, etc are garbage now, since spam and fluff posts were basically left to run rampant, driving away those who'd post thoughtful, interesting discussions.
So technically no, there are communities that stayed good with more people. But usually yes.