As I expected, here is the easy solution:
"One of these includes the question of size, with GTA V being the most ambitious and generously proportioned GTA game yet, dwarving Red Dead Redemption and GTA IV in scale.
This means that the Xbox 360 version will ship on two discs, one of which will be a mandatory install disc that will require 8GB of free space on your console's HDD. The second disc will then be used to subsequently play the game, which means no having to interrupt your GTA V experience to swap discs. Players with 4GB consoles can use a 16GB USB 2.0 Flash Drive for the install."