I think it has a lot to do with heavy coding. Since this game is being coded from scratch, all the DLC is too. Its just a heavily coded game, which makes the data larger, which turns into big storage requirements for the game. No big deal for me, I bought a 3TB external hard drive well over a year ago so I'd never have to worry about this sorta thing.
You don't know what you're talking about, right? The code is probably the smallest size of a game.
And all the people people with those silly suggestions like "just get an external hdd" don't even get the point in here. The question wasn't about a workaround for this problem, but the reason for this. A few simple questions so that even those people understand the problem: Halo 5 nearly needs 100GB. The question is why? Even games with more content or more detailed textures (e.g. on pc) don't need that amount of space.There is actually NO game on any platform that needs so much space.
And why are the updates so big? Either 343 is incompetent to build incremental updates or the update system of the Xbox One isn't capable of something like this, so that big parts of the game (which don't need a patch/are affected) must be replaced instead of just the affected parts.
100GB is ridiculous for that game.