Everybody knows the difference between DVD and CD discs, we watch movies and some video on DVDs and we listen to music on CDs (well, actually, now people prefer mp3 for music listening, but, for example, I still buy CDs).
So, to underline this difference, manifacturers have decided to have different package sizes for DVDs and CDs, so customers won't be confused. When DVDs were something brand new for people, they could easily put DVD disc into their CD players thinking that it would work. So the size of package prevent people from using discs in the wrong way.
3 posts more and I'm done for this week.