Well the last games I've played were DragonShard (1 DVD), Fahrenheit/Indigo Prophecy (1 DVD), Guild Wars (2 CDs), FEAR (1 DVD, barely filled up with lots of extra stuff) and Oblivion (1 DVD)...
And till about a year, year-and-a-half ago, PC games insisted coming out on 4-5 CDs (Psychonauts, Sims if I'm not mistaken) rather than one DVD... I guess I'm viewing it too much from the PC side... :)

But I still can't imagine what would fill up 20 gigs... I'd say that they will be filling that out just to show-off the engine capabilities etc., rather than put useful gameplay time in that...