The theory that games will be limited by space constraints in the near future has been brought up time and time again, and compression is always the counter argument. I see no reason that the same argument won't work for this idea that GTA V will be forced to go to the PS3.
On a personal note, it is interesting that a developer brings up this issue, since I would assume he knows all about compression. But that doesn't mean that GTA V couldn't come to the 360 though, but perhaps with further compression that might hurt it's quality.
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