If you have it on Steam or GOG, you should have the Enhanced Edition, which includes “Classic”, that would explain the size, given the original was on floppies! (There was a later CD release with digitized sound)
The main difference is that with the Enhanced Edition, you’ll have easier WASD and mouselook controls, the original was a bit finnicky with looking around, if you’re familiar with Ultima Underworld it’s essentially the same problems with looking around, and given that this is an RPG shooter, looking around is kinda useful!
You’ll also get higher resolutions on Enhanced, as well as remappable keys.
