With the game gone gold today, Microsoft and 343 Industries announced the availability of a 20GB update on Day One for Halo: The Master Chief Collection. The big download will include some new features along with multiplayer content, but will be missing Spartan Ops, the episodic series which was supposed to be included, until December.
According to the post on Xbox Wire:
From the start, our philosophy has been to give Halo fans the best possible experience and not compromise the quality or features of the collection. The result is that Halo: The Master Chief Collection will take up almost all of the usable space of a single Blu-ray (45 GB), and we will also issue a content update at launch that is estimated to be 20 GB. You’ll be able to start playing Campaign and more as the content is installing, with some features and multiplayer content being added via the update. Yes, the update is large, but we weren’t about to cut corners to save disc space. This ensures that you are getting every bit of Halo goodness we can fit in. Our work is not done, however, as we continue to tune, tweak and optimize the online experience to ensure a smooth multiplayer launch. This will continue right up until launch day.
[Source]: Xbox Wire: Halo: The Master Chief Collection: We are Golden [http://news.xbox.com/2014/10/games-halo-gone-gold-announcement].