That's what I was thinking. Hopefully it's combined, perhaps when you download the campaign you get both and when you download the mp you only get the mp, considering the latter is free. Although it's not far fetched to think the campaign might be 100Gb's on it's own.
On this subject, I wish Microsoft would let you download the parts you want in their games (same for all games on all services, let me deselect mp, HD 4K textures and video etc). I really want to play ODST on GP on PC, but I have to download almost 160Gb's for what is a tiny game otherwise, after it's downloaded I can uninstall the other games and mp, what use is that nonsense?! Why have a hub for all the games if it still makes me download everything, infuriating?! Not everyone has fast internet or loads of free storage space Microsoft! How ironic that you van download individual Halo games on Steam, their competitor is doing better with Microsoft's own games...