Fediverse memes

As I understand it, it's kind of both.

The Bsky Relay costs are because it's the primary relay, sure, but any relay aiming to handle a mass amount of people, as well as a variety of AppViews, will likely scale similarly in costs. This is because to try to minimize any fragmentation of experience (as one may see with ActivityPub), AT protocol relays act as a central mirror of all the personal data servers connecting to them.

It's baked into the architecture for the most part, despite some later developments of lighter pseudo-relays that try to reduce some of the overhead. From the outset they've said they only really see there being a few large-scale relays due to the operational costs.