(a)
$ du -hs fs
    3.8M fs

(b) 
$ mkcramfs fs fs.crm
$ ls -la fs.crm
-rw-r--r--    1 stevej   users     1732608 Oct  2 12:49 fs.crm

Example 1: (a) Uncompress filesystem image size; (b) CRAMFS compress filesystem image size.