According to a new tweet from Metal Gear Solid creator Hideo Kojima, the open world in The Phantom Pain is "hundreds of times larger" than the one in Ground Zeroes.
The quote:
"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."