Subscriber count is the number everyone quotes and the one that tells you least. A community of 40,000 where the best post ever hit 90,000 upvotes is a better bet than one of 3 million where the ceiling is 20,000, and nothing on the internet tells you which is which. These pages rank communities by how big a post can get relative to how many members they have, measured across thousands of them.
A community appears here once we hold at least 80 of its posts and it has at least 100,000 members, because the ranking divides by member count and a small denominator produces a big meaningless number. Ranked by upside rather than by peak alone: sorting by raw upvotes would just list the biggest subreddits on the internet, which nobody needs a page to learn. See how we measure.