In an interview with UOL Jogos, Christian Diefenbach, software engineer at Bungie, revealed that each planet in Destiny is roughly the same size as the entirety of Halo: Reach:-taken from article
"The destinations [scenarios where the game takes you] like Mars, Moon, Earth, each one is more or less the same size as the entire game 'Halo: Reach'."
He also explained that the game has a much bigger team working on it:
"In 'Halo', We worked in a team of 130 people. Now there are over 500 people working on 'Destiny'."
Bungie also recently conducted a community question night, so we're expecting that to shed even more light on Destiny.
Note that this is a small article that takes excerpts from the full interview which can be found here