Mastodon amortizes load by having lots of servers, and the load on these servers is a product of the sizes of social graphs the users of these servers have. So, the size of the overall network isn’t actually relevant, it’s how big individual user graphs grow. I doubt that’s going to change dramatically going forward because you can only meaningfully follow so many people in practice. This is one of the main differences between Mastodon and centralized platforms like Twitter where the servers have to handle the entirety of the content on the platform.

The flip side is that Mastodon doesn’t really lend itself well to things like celebrity accounts that have millions of followers. That’s likely to be the more interesting problem going forward.