This is the screenshot of the properties of all files except /home:
My whole hard disk is of ~320 GB, then how is it so big?
It is the file
/proc/kcore which for some reason looks so big. Just exclude
/proc from your folders list to get a more realistic result.
/proc/kcore specifically is a file which maps directly to every available byte in your virtual memory. It is a system file, provided by the virtual
procfs filesystem, and is not a real file.