Tune mksquashfs for smaller isos.

Bigger block size, more suitable for xz.

nixos/lib/make-squashfs.nix

···
 
       # Generate the squashfs image.
       mksquashfs nix-path-registration $storePaths $out \
-        -keep-as-directory -all-root -comp xz
+        -keep-as-directory -all-root -b 1048576 -comp xz -Xdict-size 100%
     '';
 }