I spent a significant amount of my time this year on a project to migrate hundreds of mailing lists (some are twenty years old) from mailman to Discourse, primarily because they were plagued by spam. It has been a great source of inspiration and there is indeed a great deal of code / process within Discourse to manage spam. And also mediation in a conversation, moderation when a behavior goes against the code of conduct etc.
There are multiple ways to keep things small in Discourse and achieve the higher moderators/users ratio you suggest.
- Hosting entirely independent instances on the same server
- Delegating moderation (and not administrative) rights to a selected number of people for the whole instance
- Delegating moderation rights to a group of people for some categories only
My gut feeling is that things are small when a single person can read (and understand 
) all activities daily.