Was active on Reddit a long time ago, there's a liminal band of popularity in which a service tends to offer the best experience. Enough interest to be good, not enough interest to make it shitty or incentivize abuse.
It's difficult to remain in that band particularly because at some point you have to actively fight growth, not sure HN is all that immune either. I think HN tries to stay in that band via it's archaic UI and somewhat intimidating culture.