It seems quite likely that, in the Arch ( EOS ) system at least, a tonne of that space is being used up by the package cache. By default, the system keeps copies of the packages for all software you install. This can indeed take gigs of space but it has nothing to do with your running system. A simple command purges them all and reclaims the space. You would obviously want to do this before reporting installation size. I bet he did not.