Hideo Kojima has already confirmed that Ground Zeroes will act as prologue to the story coming in Metal Gear Solid V: The Phantom Pain, but has now gone further and given us some idea of the difference in size between the two games.
"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."
You can read more about why we're excited about both parts of Metal Gear Solid V in our recent 14 for 2014 Preview.