Federation doesn’t inherently require large amounts of memory. Fundamentally, it’s a matter of selecting a list of unique servers (likely tens, maybe hundreds) from a larger set of followers (likely hundreds, maybe thousands) and sending an HTTP request to each when there’s a new post. There’s a speed/size tradeoff for how many to send in parallel, but it’s not a resource-intensive operation.
Growth beyond a few tens of megabytes was a bug in Writefreely, which is a likely-suitable option several comments here recommended.
A couple of them.
Both have RSS feeds and associated Mastodon accounts. It’s also possible to follow the latter directly with ActivityPub software that allows following users (not Lemmy). I might put up a third and talk about programming and tech.