I think it depends a lot on the federated service.
For mastodon, you follow individual users, so if there’s a million users or ten million or a hundred million, their instances will only be contacting other intances they’re federating with so it’s quite scalable.
For Lemmy, you follow communities, so every server pulls all the posts and comments the common community. This means that for an instance like lemmy.world hosting lots of different big communities, every new server hammers the one central instance.
A strategy for improving the situation I think would be to spread the load. Instead of everyone piling into megacommunities, if people spread out into smaller more tight knit communities over many different instances. Of course, this isn’t really compatible with the purpose of having communities like that.
It does seem to suggest that ActivityPub isn’t necessarily the most appropriate protocol for this purpose, even though it’s what was used because it’s the de facto standard on the fediverse.