Unlike Final Fantasy XIII, the Xbox 360 version of Final Fantasy XIII-2 will be a single disc release. Square Enix managed to fit the whole game on one disc by having event scenes rendered in real time.
Before you start a big hullabaloo about XIII-2 having less content, game director Motoru Toriyama says XIII-2 has the same volume of content as its predecessor. The switch to real time has allowed the developers to put in features like showing armor changes through event scenes.
XIII-2 also has other improvements such as the Crystarium no longer limiting character growth (you can max out your characters from the start if you wanted to) and switching your currently controlled character during battles. You will also not get a game over if your party leader is knocked out
