1080P sometime is not 1080P all the time.

Since it's not all the time, then it's not 1080P.

Sometime, people still claim otherwise.

All the time, they are wrong but don't admit to.

Movies are as big as you want them to be - there's a tradeoff between quality and size.  BluRay movies are often overkill using VC1 or h.264 in the range of 25mbit - you can do 1080p for your typical 90-100min movie in very nice quality 1080p in the space of a DVD, but you're at the limit - want to improve the quality beyond 10-11mbit or have a longer movie (and maintain 10-11mbit) and it will not fit.  BluRay (and HD-DVD) are considerably larger and offer the flexibility to double the quality, add higher quality audio, and double the length of the film (all at the same time) and still have room to spare.

As an aside, very few games - on 360 and on PS3 - are actually rendered at 1080p.  Most games are rendered around 720 - in the case of the 360 it will then scale that up using a hardware scaler.  In the case of the PS3 it will actually output a 720p signal while the game is running because it does not have a hardware scaler.  Video, like DVDs, can still be scaled up using the GPU on a PS3 - this is impractical during games because the games need all the GPU's capabilities themselves.  The idea of the Xbox scaler is to offer scaling to whatever your display is set to with zero additional cost to the game engine.

Most of what comeson a game disc is merely code that renders polygons, ai, etc. in the game. The only thing that takes up a lot of space is the textures that go on polygonal models, and the audio. However, games often use repeat textures and sound bytes, stuff like that, so they don't need that much on a disc.

Moves, however, are essentially thousands of 1080p images (and the sound that accompanies these images), each of which are unique. Games wouldn't need near that number of images (or textures, in a game's case), and they'd rarely be that large.

Imagine a ~2:00 hour movie, showing 24 1080p images a second. You'd need to save 172,800 1080x1920 on a single disc. That's a hell of a lot of space. Just look at the size of any of the pictures on your desktop for comparison.

Basically, a movie contains pre-set images. A game contains the code to render such images, which takes up far less space.

And, as others have said, the mere resolution of the game has nothing to do with file size. You can render Killzone 2 at 320x240, but it would still take up the same amount of space on a disc.

 Because DVD players don't have the CPU power to handle modern video compression algorithms.

H.264 looks great at ~1.1GB / 50 minutes.

This too, of course.

It´s not that easy. If your example is right, a movie would be about 350 GB big. Even Blu-Ray aren´t  THAT big. Truth is, even movies on Blu-Ray aren´t raw data, but are compressed.

Don't forget the "unhackable" media reason.

Actually the data that is shown is still those images even if they are compressed. Whats different is that you can compress also in third dimesion when compressing movies.

And games can be rendered in any dimension, because those 'pictures' they make on screen are actually made real time by using some objects & other stuff. Games aren't even near 1080p resolution because it would require too much from GPU:s. Of course one could make game that works in 1080p resolution, but the tradeoff would be simpler graphics.