Right now on a Prosody server I have, with the daemon process running for 127 days straight, with 5 local users, +8 remote users, 9 connected servers is running at 161MB memory used.
For ejabberd, running for over 3 weeks (after restarting for changes for Matrix bridging), 6 local users, (I don't have the other metrics readily accessible) is running at 115MB memory used.
I do host were.chat, which is registration-by-invite (solely to reduce automated registration and bots), have the domain registered until at least year 2029, have active uptime monitoring and notification, and I tend to keep a deathgrip on keeping things online perpetually, even past their usefulness (such as keeping a forum online for 2 decades now, when it died in activity like a decade ago; and it's shifting into being hosted for preservation/archival sake at this point), so it's exceptionally unlikely that anything I run is going to just disappear.