I have some old directories and files that I which to archive off. I have created archive using -p option with /usr/sbin/backup to a file. If I then do a du -g on the original directory and the archive differ as I'd expect due to using the p flag to compress the files < 2gb as part of the backup.
However if I then /usr/sbin/restore the archive it is still different size to original directory (about 50% of original). If I do ls -l the filesizes it shows appear to be the same?
Perhaps the space is used more efficiently when the files are restored. If the original files became fragmented with lots of partial disk blocks being written, then you could see a space used reduction after a restore as that would write full disk blocks and therefore use less of them.
Is this what you are after or have I missed the point? You made need to explain what you are seeing a little more.