Yep that's right, bizzare creation said that the dvd format is limitating pgr4.
They say that we won't be able to see night to day or vice-versa transition because it would have required twice the texture storage... ( strange that is ) anyway full story here :
http://www.destructoid.com/size-does-matter-project-gotham-racing-4-is-too-big-for-dvd-37131.phtml