Lemmy naturally concentrates unconnected users with similar interests thanks to reddit-style communities. Mastodon follows the Twitter style where you have to find and follow individual users to get their microblog content, and its harder to isolate certain topics or interests except across the entire service via hashtags. Individual users on their own are very uninteresting and bland.
Lemmy has fewer users but they as a whole generate more active content than Mastodon does thanks to community specialization, since the Twitter style posts require some critical mass of users following to generate interesting discussion (something that basically never happens unless you’re already a celebrity)