I think the determining factor is what the community's ultimate goal is. I would say that scale improves communities where cohesion or 'quality' can be relaxed

I think the determining factor is what the community's ultimate goal is. I would say that scale improves communities where cohesion or 'quality' can be relaxed in favour of expanding the base of people helping in that goal. An example that comes to mind is the physics community. I hear a lot of criticisms in recent times about physics around the shortcomings of string theory, particle physics, and even some criticism towards newer areas like quantum computing and topological materials. In this instance, criticism actually improves the community because it necessitates the community to review and defend its beliefs. For most physicists, the rigor and empiricism of the community is extremely desirable, and so the extra conflict is accepted. In short, I am sort of asking you to be more precise. What does it mean, to you, for a community to 'decline'?