[WreckVenom]

> I used to do a little bit of level building for IdTech3 games back in the day but it's been 20 years. I'm not totally ignorant of what's involved, just mostly ignorant. I really want to know though, what is all that data!? Textures?

Mostly. I have a HD texture pack for Quake 1 that weighs in about 15GB on my NAS. That project is from 2011-ish.

> Robocop: Rogue City takes something like 8gb.

Did you mean 38GB? According to Steam the space required for Robocop Rogue City is ~38GB.