From what I understand, they don’t take as much space as people usually think they do. The reason is that they deduplicate between the various flatpaks that are on your system but don’t make this obvious.
Storing applications in a local OSTree repository has other advantages. For example, it allows files that are stored on disk to be deduplicated, so the same file that belongs to multiple applications (or runtimes) is only stored once.
I’ve heard that the size that you see listed in the Flatpak before you download it is NOT the actual amount it needs to download or install. Instead, Flatpak calculates what you actually need to download and install, taking into consideration which packages you already have installed in Flatpaks in your system.
From what I understand, they don’t take as much space as people usually think they do. The reason is that they deduplicate between the various flatpaks that are on your system but don’t make this obvious.
From https://docs.flatpak.org/en/latest/under-the-hood.html:
I’ve heard that the size that you see listed in the Flatpak before you download it is NOT the actual amount it needs to download or install. Instead, Flatpak calculates what you actually need to download and install, taking into consideration which packages you already have installed in Flatpaks in your system.