Metal Gear Solid V: Ground Zeroes is set to act as a prologue to Metal Gear Solid V: The Phantom Pain, but according to Hideo Kojima, the latter dwarfs the former quite a bit. While discussing the pair of games on Twitter, Kojima said Phantom Pain's open world is "hundreds of times larger" than that of Ground Zeroes.
“GZ is both a prologue to the MGSV story and, at the same time, a tutorial for a new MGS, which transitions to sneaking simulator. With that said, GZ has a relatively small field as compared to the main game (which is hundreds of times larger) without time zones changing in real-time. However, each mission offers different time zones with different playstyles.”
The Metal Gear Solid V saga starts when Ground Zeroes hits Xbox 360, Xbox One, PlayStation 3, and PlayStation 4 in spring 2014.