It's now well known that Metal Gear Solid V: The Phantom Pain and prologue Ground Zeroes will be releasing as separate titles. In a recent interview with Famitsu magazine, series creator Hideo Kojima has explained the reason behind splitting the titles up, and how the The Phantom Pain has "enormous volume".
"If I were to put it in Hollywood movie terms, the prologue, that is Ground Zeroes, would be similar to the first 10-15 minutes that are meant to ‘captivate' [the audience]," says Kojima. "At first, something will happen, then the title will appear like ‘bam!' and then nine years later, a big event will be the start [of Metal Gear Solid V]. That's how the story will be connected," said Kojima (via Siliconera).
"As far as the game itself goes, The Phantom Pain will take place in an immense open world map, while Ground Zeroes takes place in a smaller open world," he added.
"It would've actually been better to release it all at once, but by the way things are going, it appears likely that Phantom Pain will be coming out around the time the PlayStation 4 becomes more spread out. That means we would've had to make people wait a while longer."
"Prior to the announcement of Metal Gear Solid V, last year, we showed a Fox Engine implementation video," says Kojima. "Then, we also showed the Fox Engine in a presentation video at GDC 2013. Since we revealed information on a regular basis, there were many who believed that Metal Gear Solid V was already complete, and there were many voices from fans saying ‘we want to play it soon'."
"However, The Phantom Pain has enormous volume, so it will require much more time," Kojima explained. "So, together with the marketing staff, we consulted ‘what ifs' and such, and came to the decision of allowing fans to at least play the prologue, ahead of time. We want to offer it for as cheap as possible, so at first we thought about making it as a download only; however, after thinking about people around the world, and how some places don't have the best connectivity, we're preparing a physical version."
Ground Zeroes has been given a launch window of spring 2014, and it will be available both digitally and at retail on PS3, PS4, Xbox 360 and Xbox One. You can read more about that here. Release details regarding The Phantom Pain have yet to be announced.