about ps3 and xbox 360 games?

ultimately yes, cross platform gmaes on Ps3 are only using a fraction of the full Blu-Ray.

But Exculsive titles like MGS4 use up the full 50GB, and LittleBigPlanet is 40gb.

However as games become more demanding, and devs' learn to use the PS3 better ( there getting the hang now, look at MGS4) then developing for that then porting to 360 becomes mroe common, ie Final Fantasy XIII, then they will use the full Blu-Ray capacity, so the 360 version will probably be multiple discs, and even then it iwll use compression.

Developers are also using hte spare space on the blu-ray to copy the game files multiple times across the disc, so this will speeed up loading times as the disc head cna read the data almost anywhere on the disc, jsut not having to go to one place all the time.

A good analogy of compression is a block of foam. If you squish it down(compress it) it takes up less space, but its not very spongy, so you need to uncompress it to use it as a spongy block of foam.

OK, this is what happens. On Xbox 360 the games need to be compressed as in 'squished together' to take up less room on the disc so everything can fit which is why there can sometimes be a slight loading difference between the PS3 and Xbox 360 but the difference is never significant because or drives spin the discs faster therefore reading the info quicker which evens it out more or less. On the Blu-Ray discs however, since there is more room, the files don't need to be squished together and the reason MGS4 takes up the entire Blu-Ray disc is because the developers decided to leave ALL the audio uncompressed therefore it took up more room. But still between the change of every chapter in MGS4 the games need to partially install itself into the PS3 hard drive and takes up a few gigabytes. However the reason other games don't start uncompressing material is because that is just what they are used to. The PS3 is more difficult to develop on than the 360 so to decrease the amount of time it takes to make the game and the difficulty of the process overall the leave the stuff compressed and don't bother themselves to take up the full 50 gigs.

Compression - Files often repeat the same piece of info over and over and compressing only takes the file part once and refers back to it after decompression when required.

Go here to understand more fully How It Works.

http://www.howstuffworks.com/file-compression.htm

no, each disc is formatted differently. most games are more "stretched" out on ps3 games, where as 360 games are more compacted, hence the sometimes longer load time. but HD drives actually spin alot faster than blu ray drives, and this makes a difference too. so dont sweat it

first off xbox 360 dosent use hd dvd hd dvd is an OPTIONAL componet

and hd dvd did have 30 gb of storage on a dl disc

second off mgs4 is a ps3 only game it not on the xbox so it prolly does use alot of that space on the bluray disc