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).
Sigh. It appears that they are chasing after the latest “shiny” thing instead of investing in existing infrastructure. Not surprising, but disappointing.