Why wouldn't it be? Most of the size in games like this come from audio/visual assets. Videos, large textures, long audio (lots of recorded audio for this game). Patches generally fix broken things like code and scripts, which take up little to no room at all. Some patches change assets, but not many do. So that is why a game would have relatively small patches for a huge game. On the other hand some patches are huge because the game can be stored in compressed archives, and rather than unpack the archive that contains the changed data, replace the changed files and repack, a lot of developers opt to pack the archive over again on their end and let you download and replace the original.