Mastodons community is also very much aware of the issue. There are some who even call for defederstion from large instances. The main impetus being mastodons people’s concern for safety and moderation, where big instances necessarily allow much more to pass through. So while there’s more of a push from the top to make mastodon.social the flagship, from the grassroots there’s very much a push against it (I’m loosely a voice in that might self).
Lemmy.world’s push to be a big vanilla instance is not much different from what’s happening over at mastodon I’d say not least because it’s run by people running one of the big mastodon instances.
In the end I’m not sure the two spaces differ that much in their dynamics around this, notwithstanding the differences in the attitudes of the lead devs, especially given that the same result has occurred. Which is why I say it’s likely a natural phenomenon. You’ll found the distribution of instance sizes likely follows a power law which is common in nature.