mkisofs increasing iso size by 100 MB

[Amitabh Kant]

I am trying to customise the bsdinstall auto script. I can mount the iso
(amd64 arch / 9.0 RELEASE) and change the shell script as per my
requirement. Once I try to re-create the  iso file using mkisofs utility,
the size of the final iso increases by 100 MB if -J (joliet) mode is used.
If I remove the joliet mode, it still increases by around 97 MB. Even if no
changes are made to any of the files, the result is same.

The process I have followed is as follows:
# mkdir /usr/iso
# cd /usr/iso
# dd if=/dev/cd0 of=org.iso bs=2048
# mdconfig -a -t vnode -f org.iso -u 0
# mount_cd9660 /dev/md0 /mnt
# mkdir staging
# cd staging
# rsync -a /mnt/ .
With Joliet mode
# mkisofs -J -R -V CustomBSD -no-emul-boot -b boot/cdboot -iso-level 3 -o
/usr/iso/my_custom.iso .
Without Joliet mode
# mkisofs -R -V CustomBSD -no-emul-boot -b boot/cdboot -iso-level 3 -o
/usr/iso/my_custom1.iso .

The original iso is 612M, custom.iso is 712M and custom1.iso is 709M.

System details: FreeBSD amd64 9.0 RELEASE running inside a virtualbox with
2GB RAM.

Where am I going wrong?

Amitabh