If DVDs are 17GB, why can the writers only burn 4.7GB? Is it true that isnt enough to copy a DVD movie without the special features unless you also sacrifice quality by lowering the bitrate?

I was just about ready to buy a DVD writer so I can copy DVDs and keep the original quality (I just got a projector and 72" screen so it will make a big difference) but a friend told me only old DVDs would fit on the 4.7GB of DVD-Rs without lowering the bitrate.

So is it true that even by removing the special features new movies wont fit on 4.7GB without sacrificing quality?

Thanks