lemmy.world is #2 by total user count (lemmy.ml being the 1st), but #1 by active users.
And judging by the Local posts and Local comments count, it seems that .world users interact more with communities in other instances than the local ones, unlike the other top instances.
So I would dare to say that your concern over content being monopolized by .world (based on subscribers to local communities) isn't founded - high number of users, but they tend to subscribe and interact more with communities on other instances.
This is of course anecdotal (same as your example), but I tend to see the opposite in my feed - few posts from communities on .world. It's very subjective based on what you subscribed to.
The high number of users on .world is because it still has open registration (server was recently upgraded beyond current capacity).
Edit: I briefly forgot that kbin uses a different source code / platform than Lemmy, even though still uses federation and ActivityPub and counts as part of the fediverse. So it's counted separately here.
Same like Mastodon, Matrix etc which are different platforms that have the ability to federate with some of the other fediverse platforms like Lemmy.
And also just now I learned that kbin has multiple instances, so it's like its own thing, not just a Lemmy-federated instance.
Interestingly, Active users ~= Total users on kbin.social. But I have no idea how that Active number is computed for either lemmy or kbin on that site.
Or joining one gym and going to another gym of the same franchise. Just like I'm doing now, hello from kbin :)
I've seen a lot of kbin users interacting with the rest of the fediverse, I think because it has a better sorting, it's easier for us to find new threads on other instances.