I like the smaller forums too, I belong to a gardening site that is fairly busy but not overwhelming, it's part of a much bigger website but they have sub-forums, the one I'm on focuses on Florida gardens.
I like the tools and categories this one has and the friendliness of the members, some of the other pro forums pretty much ignore you if you're not one of the old dogs.
I belonged to a dog and cat forum for a while that only had about twenty members but it was busy as could be, good discussions and no trolls.