I think it might have to do with developer/debug mode being enabled during building. I don't know how godot works, but in Unreal and Unity the file size of your build is increased if you have developer/debug mode turned on. I think it adds additional binaries/packages to the build that release builds don't include, thus inflating the file size. My main suspicion for this being the reason is that I noticed the title bar of the application said [DEBUG] after the name.