Could be what communities you are subscribed to. I run a small instance with about 3ish users, and here are my stats after about 3 months as well:
9.5G ./pictrs
12G ./postgres
8.0K ./lemmy-ui
What version of lemmy are you using? A recent update also introduced some space savings in the database (I think).
I think the “Ubuntu Core 22” means it is the snap based version of Steam rather than the deb version.
If you look at the snapcraft.yaml for the Steam snap, it uses
core22
as its base.