On the picture the shadows of the buildings are just looking different because the object casting the shadow is farer away. Their system works like in the real world, the shadow of an object far away from the ground is much more blurry than the one which is close to the ground.
Doom3 actually...
The 12 MB it tells you is not any size of HL2 Media.
The Problem is that it isn't available yet, so the title is also wrong. It says "Platform" and that are the Steam files. Which means, the size shown is just the size of the main Steam engine files.
I also think that there will be more on...