davidoc : This is an obstacle, but we can find out the extra MB added just for Mono and subtract it from the Mono projects.
xudz : .pck (in pure godot engine) is just all the content scripts, models etc. For example export a project named for example one. It will create an one.pck and one.appimage (exe or what ever). Afterwards export another project named two.
Go delete one.pck and rename two.pck to one.pck and then run the one executable. The two project is going to show up.