The issue is memory. Every library in an app image has a unique memory space and so you have a bunch of copies of sometimes very large libraries sitting in memory rather than one copy from disk mmapped into memory and page duplicated all over the place.
Linux has had support for not doing duplicate pages for a long time now. I am forgetting the name of the feature but essentially this duplication is a solved problem.
That's only the case if the libraries loaded are identical, It won't work with slightly different versions of the same library (unless the differences are small and only replacements, so the pages remain aligned between different versions), and that case is very unlikely to be solvable
Could be big, depending on how much room you give to /. All my Linux life, I have allocated about 50GB to the root partition and it's been adequate, leaving enough room for my data (on a 512GB drive). Now I install one flatpak and I start getting low disk space warnings.
- Qt 6 (core, widgets, gui, network, qml, qtquick, serial port, websockets and a few others) and all its dependencies excluding xcb (so freetype, harfbuzz, etc. which I build with fairly more recent versions than many distros provide)
- many random protocol & hardware bindings / implementations and their dependencies (sdl)
- portaudio
- ysfx
with Flatpak I'd be looking at telling my users to install a couple GB (which is not acceptable, I was already getting comments that "60 MB are too much" when it was 60 MB a few years ago).
My steam library is on another drive (I have multiple O(TBs) large spinning-rust drives on the desktop). The nvme is strictly the base system + apt packages + build tools + /home/.
I also shun the snap bullshit. But TBH I haven't divided my disks into more than one partition (except from /boot and EFI stuff) for many many years now.
In addition to memory, there's the ability to patch a libz bufferoverflow once, and be reasonably sure you don't have any stale vulnerable copies still in use.
