In any case, that shouldn’t apply to uncompressed data - or so I assume.
But wait, it gets better.
So after some more fiddling to get rid of minor needless bits (but always keeping everything uncompressed) I got a few MB smaller, enough to result in a beautiful final image (MDS) backup just 1MB under the 4700xxxxxx size
I open that same backup image in DVDShrink… drums… now DVDShrink reports a disc with 4481MB in Full Disc mode, i.e. 16MB larger than it actually is. If I were to perform a backup with Shrink from this backup, I’d have to compress something or delete something from it again
Maybe I’m stupid, maybe I’m just missing something still, but at this point I can only assume there’s something about file systems… clusters… file formats… and maybe a potential weird calculation bug as suggested… going on here.