Some of the structure is on the disc. Given parallel development, this is not at all surprising. Doesn't mean the content files are there, and even if they are that doesn't mean that the content was cut.
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You're correct- only the game world is on the disc, the DLC contains the content- enemies, bounties, rewards, qyuest, PS3/XBOX themes, etc.. and the key to unlock it all.
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Possibly. It's also possible there are maps and such not on the disc (or only partial maps, or content in incomplete but patchable states, or any number of other possibilities).. Trick is, it's kind of hard to know, and the reference to the size of the content on the Xbox Live store is sort of worthless given that we already know that Destiny does its downloading in-game.