Disclaimer
Flatpak uses OSTree, like Fedora Atomic Desktops (Silverblue, Kinoite etc) and similar to BTRFS snapshots.
So many files are deduplicated and linked, not actually there
https://gitlab.com/TheEvilSkeleton/flatpak-dedup-checker
50GB without
31GB with deduplication
21,4GB with BTRFS compression
I have all apps I use daily in the appimage format. Yesterday I decided to try btrfs for my root partition and did my annual Linux reinstall. All my apps were already there and ready for work from the start.
I also have a usb flashdrive always on me with the same appimages. Just in case I’d wipe a hard drive by accident and wouldn’t have an internet connection or something like that (in case of emergencies). You can’t do this with flatpaks or snaps.