I've seen dozens of examples of this, but I'm still hesitant to brand this to be causality. There seems to be some degree of correlation though.
Smaller communities tend to be more familiar - people recognize each others names, and people have each other's backs, when someone is trolling or something. When things grow beyond a certain size - maybe related to Dunbar's number - the group dynamics change.
In some cases this can lead to the community to loosing coherence or even becoming toxic - but I don't think that's inevitable. I believe it IS possible to grow a community to a larger size and have it stay nice. I think though, that in order to keep a community nice when growing beyond a certain point, some effort is required - effort which wasn't necessary while the group was smaller and kinda self-organized.
Also, growing slowly rather than quickly might help - as newcomers have some time to acclimatize and internalize those unspoken social rules that govern a group's dynamic - and will be able to pass on the knowledge to those coming after. People who join a group will eventually adapt to the group - but that process needs a bit of time.
But when growth is too fast, a newcomer might have more interactions with other newcomers rather than long-standing community members, and thus will adapt to something quite different.
> Smaller communities tend to be more familiar - people recognize each others names
It feels like there's a certain number (maybe Dunbar's like you said) at which people stop seeing a community as a group of disparate individuals and start seeing it as some sort of a collective hivemind.
You see this regularly with comments like "HN/subreddit/etc is so hypocritical. One day they're in favor of X, the next day they're against it", in which the poster seems to fail to realize that in a community of tens of thousands of people, chances are it's entirely disjoint subsets of the group taking those positions.
I don't see that kind of thing as often in smaller message boards. When you recognize everybody's name (or at least their avatar), it's a lot clearer that it's just Bob and Alice who support X and Eve and Mallory who don't.
It seems natural that such a failure to see the individuals that make up a group would lead to a decline in civility.
Smaller communities tend to be more familiar - people recognize each others names, and people have each other's backs, when someone is trolling or something. When things grow beyond a certain size - maybe related to Dunbar's number - the group dynamics change.
In some cases this can lead to the community to loosing coherence or even becoming toxic - but I don't think that's inevitable. I believe it IS possible to grow a community to a larger size and have it stay nice. I think though, that in order to keep a community nice when growing beyond a certain point, some effort is required - effort which wasn't necessary while the group was smaller and kinda self-organized.
Also, growing slowly rather than quickly might help - as newcomers have some time to acclimatize and internalize those unspoken social rules that govern a group's dynamic - and will be able to pass on the knowledge to those coming after. People who join a group will eventually adapt to the group - but that process needs a bit of time. But when growth is too fast, a newcomer might have more interactions with other newcomers rather than long-standing community members, and thus will adapt to something quite different.