I don't think it's the DVD drive that is making this bigger. All those parts needed for a comp requires around 7 inches already and they may be just decided to have a DVD drive too.
Parts can be shrunk (see iphone, ipad) but a dvd has a fixed size you can't shrink so there you have the weakest link.
Btw, it was 3x2x1" what I meant. The perfect size.