Sanunes said:
the answer to that is bad file storage. they probably went the easy way and stored the data in large archives. like "textures.dat holding ALL textures in the game in one file, which actually has 10.000 files inside, but hard drives treat it liek one file and thus less likely to get things fragmented. however they probably encoded the archive in such a way that you cant replace part of it without reprocessing whole archive (part of why modding some games were hard). so they thought its easier to jut redownload whole archive even if it changes only part of it, hence the large sized download. we see this a lot in the MMOs and their archived packages. expansion comes out, heres a 5 gb download that wont actually icnrease the size of the game.
Basically this is good idea when you plan to never update the game as it usually causes less trouble for people that dont know how hard drives work and get it fragmented (as NTFS actually looks if it can fit whole file without fragmenting when writing, which causes less fragmentation when using the drive, and FAT 32 doesnt (but really noone uses FAT anymore))