Absolutely nothing wrong with that at all. A very nice burn.
Well then it could be down to the transcoding/encoding within Nero. It might help if you used 100% quality ie no compression within Nero (if that's possible) then compress to fit with DVD Shrink or CloneDVD2.
The fact that you've only got a DVD of 3.5GB , which I guess was down from one of 4.5GB , suggests that Nero has done some compression & possibly not done it too well.
Bear in mind that this is not my area of expertise - not sure what is - so I might be talking out of my a**e.