As @Masimatutu points out, for a community on another instance, you normally only see the subscriber count of people from your own instance who are subscribed there, which can make other instances look smaller than they are.
If Lemmy could have some inbuilt support for tags or subscribing to a topic / multi-instance community,
I think you are right. Kbin now has multi-communities called Collections, and it's a bit of a gamechanger.
We can make our own and/or follow other people's public multis on various topics. I am now seeing so much more content from way more instances.