While modders and "repackers" can reduce game sizes for easier downloading, there are hard limits to how small the files can get while remaining functional:
If you want a legally gray but not yet full-size download, scene groups like FitGirl, DODI, and ElAmigos produce of games you already own. FitGirl’s GTA V repack is ~35 GB (not 100 MB). After installation, it expands back to 65+ GB. This still requires a legal copy of the game. download gta v highly compressed 100 mb for pc full