Yup, it's true that sliding sync proxy increases disk space further, although it's not so bad. A big advantage once it's stable and merged into a native homeserver will be reducing the storage duplication between the proxy and the HS.
I just went through a HUGE pain in the ass with my homeserver. I was on the Hetzner cloud plan and at the highest storage tier already. I had about 80 GB free, and setup sliding sync. It filled up the remaining space (unbeknownst to me) and my homeserver stopped working. Fortunately I was still able to login, but no disk space was available for the Postgres vacuum. I wound up having to rsync everything over to my desktop, setting up Docker on my desktop, running the compression tools on my desktop, and re-uploading. I now have a 20GB dummy file on that server.
Meanwhile, we really need to get https://github.com/matrix-org/rust-synapse-compress-state/pu... hooked up in Synapse so that it automatically compresses state; I don't know why this never landed when it was written in in 2021 :(