Thanks ch for providing numbers for the compression-ratio:

2.4G        grml_chroot
-rw-r--r-- 1 root root 698M Nov  7 20:44 grml_isos/grml_0.0.1.iso

So, this means a ratio of 3.5 : 1.

If I summarized correctly it's 2.25MB (other WM removals, w/o fvwm packages).

2.250 : 3.5 approx 643 [kB]

That's not much!

fvwm2 + fvwm2-crystal would save 5.6MB (squashfs-compressed)

20.156,8 : 3.5 = 5.759 [kB]

- Sedat -