By audiovisual standards it's very, very light.

Indeed, and non-compressed video is even worse. The PS3 notwithstanding, most commercial-quality A/V tends to compress by 80-90%, often with no perceptible loss of quality at all. 50% is nothing.

i don't understand the problem with them compressing data or substituting storage for something else. in the end the games will be directly identical to one another and all fanboys or reviewers would want to do is pick which version is the best over the other.

but from a mainstream, casual person like me. i personally wouldn't tell the difference and would buy whichever version i wanted.

but if it was a direct piggyback version that square enix is developing for the 360 then i would understand but by the looks and reads of this thread they are only taking a little bit away for something else.

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Marketers have spent a lot of money trying to convince media consumers that they can tell the difference between well-compressed A/V and uncompressed. This is not unique to Sony fans, though the PS3's positioning as a media center has caused a lot of the BS to leak into its marketing. Therefore, Sony fans react to compression as though it would significantly degrade the game, when the fact is that it simply will not.

Compression

but it will not. all square enix wants to do is make both games equal. it would be happy for sony fans to point out to others that "oh man look the ps3 version is better because its the RSX graphics chip thats making this happen or the Cell processor" and if, well square enix is doing this with the ps3, subside one their version to make identical sony fans would say "i don't understand why they would do that to make us compared to the 360 version, not my fault their hardware can't compete with ours". 

you see in the end all hardware is the same, nothing is better than the others. not one mainstream casual consumer would even care which version is the best. all they see is final fantasy 13 on the ps3 and xbox 360 and they will purchase it on whichever console they own. 

personally im not buying the ps3 version because of graphics or superior technicial features over the 360 version that some of you will take in consideration. im buying the ps3 version because that's where final fantasy has been for the past 2 generations and it fits in my gaming collection.

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Could be that the 360 version has alot of compressed data on the discs witch need to be installed to the hard drive. So the question then is, how much hard disc space is this game going to eat up. FFXI needed the hard drive aswell so it's definitely possible.

I thought MS had a mandatory 'no disk required' policy?  Wouldn't that prevent them taking this approach?

Only in your world dude.

not this...