Those are repacks, a repack can often refer to software that has been modified in some way to make it smaller.
Basically what they do is take that 10 gig game and remove heavy files that are usually useless, some sound and video files have no impact on the game, so those gets tossed.
Only the essential stuff to run the game and play it is left in the repack, they do not have a compression method that will halve the size of the game, its basically just a bare bones game. This is the usual repack in terms of pirated software, other repacks increase size by adding addons and mods.
As far as i know, to keep the file from getting corrupt after compression you still need to retain ALOT of its integrity in the code, so i doubt that a compression of 50% is even viable for anything software wise, perhaps someone with more recent experience with this can answer more precisely, but thats what i have