Sorry I can't help with the problem, but I've had an instance when DVDFab would not shrink a Dual Layer disc down to Single Layer - and it had nothing to do with miscalculating disc sizes. DVDFab just would not shrink it down. It made no difference which mode was selected: clone, main movie, custom, not the output: to DVD, to ISO, or to folder, nor which burning engine that was chosen. Note: The main movie was 5936MB in size, so it had to be shrunk to 72% to fit onto a DVD-5 disc.
The happened with a DVD of the French film: "Ã l'Aventure". A UK release from: "Axiom Films" with disc ID: 263D5BF6787A8C32 [A _LAVENTURE] under EAN/UPC: 5060126870371.
However DVDFab would only ever produce a full DVD-9. The logs showed only that a Dual Layer disc was needed with each attempt - despite DVD-5 being specifically selected every time within DVDFab. I tried reinstalling, wiping out registry settings, and earlier versions, but that disc just would not shrink.
Unfortunately, I never did discover the cause of this problem and resorted to copying to DVD-9 for the friend that wanted to borrow the disc. No other DVD discs have exhibited this problem.