How to prove the accuracy of an iso file built with dd from a DVD

Question

I am trying to archive a data DVD that came with my scanner and am unable to get an exact copy. An iso file of a DVD burned from the iso file of the original DVD is LARGER.

Copy errors do not account for the problem. Everything I have tried has been repeated and proven reproducible. Many references imply that the dd command is indifferent to formatting schemes and can therefore (I think) be used to duplicate optical media, audio CDs being an exception.

The following commands have been run both in my account and as root (superuser) with no difference. (Fedora 30)

With the original DVD in the drive:

dd if=/dev/sr0 of=origDVD.iso bs=1024
cat origDVD.iso        |sha512sum >sha512sum.origDVDiso.txt
dd if=/dev/sr0 bs=1024 |sha512sum >sha512sum.origDVDdsk.txt

Put blank DVD in the drive and burn it from origDVD.iso

growisofs -dvd-compat -Z /dev/sr0=origDVD.iso

Close the drawer with the copied DVD in the drive:

dd if=/dev/sr0 of=copyDVD.iso bs=1024
cat copyDVD.iso        |sha512sum >sha512sum.copyDVDiso.txt
dd if=/dev/sr0 bs=1024 |sha512sum >sha512sum.copyDVDdsk.txt

• Results
  • MATCH (as expected):
    • sha512sum.origDVDiso.txt AND sha512sum.origDVDdsk.txt
    • sha512sum.copyDVDiso.txt AND sha512sum.copyDVDdsk.txt
  • DO NOT MATCH (which is the mystery I need help to understand):
    • origDVD.iso AND copyDVD.iso
    • sha512sum.origDVDiso.txt AND sha512sum.copyDVDiso.txt
    • sha512sum.origDVDdsk.txt AND sha512sum.copyDVDdsk.txt
  • iso file Sizes
    • origDVD.iso 2348851200
    • copyDVD.iso 2348875776

The copied DVD mounts and appears to function correctly even though it is provably different from the original.