Frank O'Connor from 343 Industries has apologised for the first huge 20GB patch which you had to download right at the first day.
But it wasn't so hot as it seems. Some fans were criticizing this annoying behaviour of the game but it was meant simple for multiplayer part of Halo. Therefore you can easily play single-player campaign and don't bother about the download in the background.
And as he also said it was not practical for this product to put it on 2 discs right now and he understands the irritation of some players and hopes that the final package will speak for itself.