hi everybody

i am trying to remaster quantian dvd v-3.6
the total dvd is 1.9 gb but.
when i am trying to compress the file with 'mkisofs' in chrooted environment
the origenal dvd is 1.9 gb but after compressing i ama getting 1.1 gb
i haven't removed any thing from the original dvd
and when i tried to check the compressed file
i am not able to boot with it

so what is the reason?