Well as far as I know Carmack is pushing new tech with this engine (Called megatexture), with the purpose of making more realistics enviroments, probably thats why he said it (Two dvds with tons of huge textures is not wnough for him), the interesting part will be how big is the difference...
"The demonstration with which the new game engine was shown had 20 GB of texture data (using a more advanced MegaTexture approach using textures with up to 128000x128000 pixel resolution) and a completely dynamically changeable world."
Edit: Some people need to understand that this case in particular is due to the Tech of the new engine, and this is only the wet dream of Carmack, he want to put the Tech5 as a new and competitive engine (Like UTE or CryEngine2), so far this is the only game using it, and probably there is not going to be many of them... like Tech4 (Doom4,Quake4 and Prey)... is a hit to the 360 yes, but is not a big hit...