Hi Gary,
I can answer you question: why the IFO filesize may change after ripping?
Since
DVDFab Decrypter will read IFO file into DVD structure, then fix the invalid part, finally write new DVD structure into new IFO file, so that the filesize may change.
DVD Decrypter will simply copy IFO files, so that the filesize will not change.
Best Regards,
Fengtao