I work for reddit and here I’ve used a similar technique to your jaccard distance but with one twist: divide by the size of the smaller subreddit (in your case, the number of unique posters that you’ve recorded). That gives you a directional relatedness, that is programming->python but not necessarily python->programming. Used this way you account for the giant subreddit problem automatically but now the results are less “amitheasshole is related to askreddit” and more like “linguisticshumor is a more niche version of linguistics”.

The great thing is that it’s actually more actionable as far as recommendations go! Everybody has already heard of the bigger version of this subreddit, but they probably haven’t heard of the smaller versions. And it’s self-correcting. As a subreddit gets bigger we are less likely to recommend it (which is great because it needs our help less)