Borderlands 2 is "50-60%" bigger than the original game thanks to better technology.
The length of the game and the scale of the cities are so much bigger because the technology wouldn't allow it in the 2009 original, Gearbox VP Steve Gibson told Games Master magazine, via CVG.
"I wouldn't be surprised if it was about 50-60% bigger [than Borderlands 1]," said Gibson. "But it's also so much denser. Heck, just the city of Sanctuary, something like that couldn't exist in the first game. Our technology has evolved a lot to enable us to do that now."
Furthermore, Borderlands 2 has been designed so it can be played even if you didn't play the original game.
"A decision that was made at the very beginning of the game was that at no point at any time could someone in the story or design team ever assume that some person has played Borderlands 1. You just start it from scratch."
By Ben Salter