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
The appimages I used dont autoupdate. But even if, you are in some weird “windows is bad” state from years ago, before the MS Store, and even without desktop entries.
There simply is no reason for appimages other than on systems like Tails that are not made to install apps. But I also think Tails is pretty annoying and should allow flatpak installs in the permanent storage partition.