You can check stats on instances here.

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).