Small private trackers usually share some sphere of interests, like music, hd tv, sports, books, manga, etc, so the volume of content is comparatively small and other people on the platform are semi-interested in seeding uploads they don’t use themselves and inspecting if they are alright. We don’t have numbers for a general-use tracker just yet. And also - most instances would be very cautious of federating contents promoting direct links to any sort of piracy.
Thank you for sharing!