It’s likely the Docker images, and maybe the Docker build cache if they built from source instead of using the Docker Hub image.
I’ve been up for about a day longer than OP, and my Lemmy data is still under 800MB. OP either included non Lemmy data in that math, or is subscribed to way more communities than me. My storage usage has been growing much faster today with all the extra activity, but I won’t have to worry about storage space for about a month even at this rate.
And that’s assuming Lemmy doesn’t automatically prune old data. I’m not sure if it does or not. But if it doesn’t, I imagine I’ll see posts in about 2-3 weeks talking about Lemmy’s storage needs and how to manage it as an instance admin.
EDIT: Turns out ~90% of my Lemmy data is just for debugging and not needed: