Interesting as a technical challenge, but practically useless for real play. If you want a tiny tactical shooter, look at Counter-Strike 1.6 (≈200 MB) or AssaultCube (≈100 MB) instead.

It sounds like you're referring to a version of Counter-Strike: Source (CS:S). These are often shared on forums, file-sharing sites, or YouTube videos claiming to reduce the game’s size from several GB to as little as 100–300 MB .