I've figured it out: I had modified the "title" and "subject" fields with PDF-XChange...
Probably the compression in NAPS2 is not optimized (I can easily manage to obtain...
I have taken the liberty to expand your suggestion, and I've made a tiny batch script...
Error importing PDF
Just tried it, works like a charm! I didn't even know there was a command line interface!...
Try ScanTailor: http://scantailor.org/
Timed batch scanning for books