For example, I might self host a server just for my account but I read all my content from lemmy.world. Am I not using their bandwidth and their resources anyway?

Well, it’d use your CPU to generate the webpages that you view. But, yeah, it’d need to transfer anything that you subscribe to to your system via federation (though the federation stuff may be “lower priority” – I don’t know how lemmy and kbin deal with transferring data to federated servers rather than requests from users directly browsing them at the moment, but at least in theory, serving the user browsing directly has to have a higher priority to be usable).

But what would be more ideal – and people are going to have to find out what the scaling issues are with hard measurements, but this is probably a pretty reasonable guess – is to have a number of instances, with multiple users on each. Then, once lemmy.world transfers a given post or comment once via federation, that other instance stores it and can serve up the webpages and content to all of the users registered on that other instance.

If you spread out the communities, too, then it also spreads out the bandwidth required to propagate each post.

As it stands, at least on kbin (and I assume lemmy), images don’t transfer via federation, though, so they’re an exception – if you’re attaching a bunch of images to your comments, only one instance is serving them. My guess is that that may wind up producing scaling problems too, and I am not at all sure that all lemmy or kbin servers are going to be able to do image-hosting, at least in this fashion.