There's a sweetspot where the sub is big enough that there's activity and content of interest but not so big that it just turns into a mess of low effort upvote farming.

There's also a few different sub archetypes that have formed. For instance /r/nfl is a low effort upvote farm due to its size, but the moderators do a fantastic job of policing submissions. So if you use it as a news aggregator, it's great. Just don't read any of the discussion.