For some reason Creative Assembly decided that they would include all the DLC in the base game files (even if you don't own the DLC, including the Fall of the Samurai Exp) and add in the numerous updates etc and you need to download a 13GB update. The game takes up 15.7GB on my machine and I would still need to download a 12GB update to play. It's not really a problem with Steam.
@TheVoxelman on twitter