Discoverability is an anti-feature. Small groups are good because they have a small number of long term users who you get to know. Everyone is less inclined to troll because they are emotionally invested in the group and being on good terms with other users.

How you end up in these groups is you either get invited by real life friends or you hang around in a big public group for a while, make friends with people who will then invite you to these smaller groups.

Because users have to be invited rather than just finding the group on a search page, the users are of higher quality and share similar interests.