About the size of these games...

I was talking with some friends earlier, and I have a friend who's getting hold of Air for me (I'm very excited, even though I won't be able to understand it ^^; I can look at the pretty pictures and go "ooh aah")... and it seems that it's on two disks, a blue and an orange or something... and it made me wonder (and I thought maybe someone here might know), what is it that makes these games so big?

Like, one of the places me and my friends often look at online has like, games for download and stuff (won't say much more than that, sorry ^^;)... okay when the games have voices and such, voices'll probably take up a good 400mb or so at a minimum... but when it's a game where there's just pictures and music and sound and stuff, how can it take up so much space? Like with Air having two disks... what is there in the game that makes it two disks? @_@... And wasn't Clannad on a DVD disk, despite not having voices? I'm cuuuuurious.