faire le menage:

- List all installed flatpak with installation type, size and application ID.
flatpak --columns=app,name,size,installation list 
- View only the flatpak installed by you.
flatpak --columns=name,size --user list
- Use the following command to uninstall flatpak packages that are not in use. This is a safe command which you can try.
flatpak uninstall --unused
- Finally, there are some flatpak cache files in path /var/tmp/flatpak-cache-*. Although, the size may not be significant. But you can still remove them.
sudo rm -rfv /var/tmp/flatpak-cache-*