A "repack" is not a modification of the game’s code, but rather a highly compressed, re-packaged version of the installation files, usually distributed via torrents. While the official Steam installation of Civilization V (including all expansions and DLC) can sit comfortably between 6 to 8 GB, a highly optimized repack can shrink this footprint significantly for transfer, only to unpack it back to its original size on the user's hard drive.