It's not just you or just nero for that matter, every program I've ever used has done the same thing, and if you use something like dvdshrink you'll notice that it shrinks down to about 4.3-4.4 GB not 4.7. I always assumed that it was something like what happens with HDD, where the size on the box is not the same as the actually data size. 4.7GB ~ 4700000000 B = 4.377...GB. That's just what I always figured, dunno if it's right or not. I've never actually thought about it much.