I always rip to DVD filesets and don't use ISOs.
When you rip to a fileset it's usually slightly smaller depending on what has been removed.
With so many protection variants though every disc is potentially different and may or may not need material removed, so you never really know what is going to happen with a particular disc until you try it.
If your ISOs are all the same size though then it must be something to do with the way DVDFab handles ISOs .
DVDFab will have removed all the protection from the files within the ISO but from what you say it sounds possible that DVDFab is perhaps padding the ISOs, but I'm not sure on that one.
It's nothing to worry about though.
Magic DVD Copier is indeed very good and I was impressed when I reviewed it recently.
You may also find Ideal DVD Copy interesting which I also enjoyed testing.
If you'd like to read further on either of these programs check out the review links in my signature.