For example, /r/mildyinteresting has 286k subscribers. Lemmy has around 51k active users across the entire platform and all of it’s communities. And /r/mildyinteresting is a misspelling of the even more popular /r/mildlyinteresting (23.4 million subscribers).
Even some of the niche gaming communities are larger on Reddit (/r/Kenshi, 145k; /r/factorio, 370k).
I would take those numbers with a grain of salt. Reddit tends to artificially pump these numbers, for instance geodefaults have millions of subs but only a few hundreds actively participating