From a lifetime of small message boards It's easier to drive engagement in smaller groups. If there's less overall exhaustion with the basics in any niche, splitting the new members is a good way to keep differentiated material. Also growing communities can end up boxing out their regulars. It might be hard to get started, but the small communities tend to be resilient at some point, they just migrate service to service.

Most of the people who moved here were especially motivated to overcome the barriers to entry to, so I'm not sure the numbers still hold.