Why second parts are smaller ?

[Xtraño]

Last weekend I was doing some refreshing on my xbox HDD. And I noticed something about two games. Hunter: The Reckoning size is almost 3 GB, but its second part, Hunter: Redeemer, it´s a little bit over 1 GB. Somethings like this happens with Dead To Rights. The original game was over 2 GB and the recently published it´s a bit over 1 GB too. If you look at the games thinking about "first part" and "second part" and the time between releases. You´ll see better textures and graphics (mostly with Dead To Rights). The point and question is. Why the second parts, which usually are bigger and better, are smaller.
Developers learned how to optimize textures or the way to progamming ? Both of them they´re not heavy FMV use.
I´ll be glad to read your comments.