My hunch is that it's not so much about size, but the rate of growth. What large communities I've seen that have managed to maintain their identity have either been so esoteric as to not have a lot of growth (their numbers having accumulated over a large amount of time), or some aggressively maintained expectation that newcomers should lurk before they post.

I do think it makes some sense, if you have a sufficient influx of new members, a large proportion of those you interact with will be strangers, and that's fundamentally different from a group where most people you interact with are familiar faces.