BD Ripper (3D Plus) Why is the MKV's file size smaller than the original ISO's file size?

[nickff]

I originally used DVDFab to create an ISO of "A Quiet Place". The ISO contained the main movie, all English audio tracks, and all English subtitles. I then used that same ISO file and had DVDFab create an MKV using the passthrough feature. I kept all the same audio tracks and subtitle tracks, but the file size was approximately 2Gb smaller. Why is that? What was "lost" in the conversion?

Is there almost 2 Gb worth of data that is needed for the ISO file to be playable in certain situations (such as creating another physical disc for playback in a Bluray disc player)?

[Mona]

I will get the developer to check your report.

[Mona]

UPDATE： The ISO source contains non-movie data, when ripping with DVDFab, only the movie part is converted.