If it's done while putting the game together, it's very little work. Now, if you're given a giant game that uses like 60% of the files and you have to read every map and most of the database to figure out which parts were actually used, then yeah, it'll be a long, long time.
Yeah. I'm too lazy to contact them, but anyone's welcome to tell them and take credit for having found the bug.
tl;dr game length ≠ resource size
It's a combination of design trade-offs, coding, resource reuse, and other factors.
Skyrim comes with some pretty low resolution textures out of the box because they also targeted consoles. Higher-res, fan-released textures, probably without full game coverage, are about 2.2GB. Bethesda figured—correctly!—that those would be "good enough" for people to buy it on PC, even if their game doesn't get them heralded as the new Crytek. Skyrim's PC version also had an idiotic RAM cap appropriate for consoles, for a while. Thankfully, Bethesda releases all of those mod tools, and has a player-base dedicated enough to fix lots of their games' shortcomings.
World of Warcraft installs, with all expansions, are probably >20GB by now, because of sheer content quantity. I imagine if Blizzard could have shrank it much without making sacrifices, they would.