Most storage space viewers get confused by Flatpak's heavily deduplicated and compressed files, leading to them reporting way larger space than what's actually occupied on the hard drive.
It's true lol. I had to install Docker for teaching on my old drive and that instantly maxed out my root partition even when I kept deleting intermediate builds and unused data. Now I have this fun paranoia for all apps :)
First of all, stop using legacy SI units for the size of information, they only bring confusion, instead use IEC/binary units like GiB. https://en.m.wikipedia.org/wiki/Binary_prefix
Second of all, I know that with Flatpak's ease of installation/runtime comes great size hit. It's great that some layers are reusable, so it's not a huge hit. Besides, with big disk size it's not really a concern now is it?
Normally sed just passes along the edited text to STDout (printing in the terminal usually).
With the -i option it actually changes the input files. If you add an extension immediately after the -i it apparently makes a backup of the original with that extension.