For those of you freaking out about the size of DS2, relax a little! A lot of things can influence the filesize of a game. The original Darksiders was the first game Vigil ever made, on Darksiders 2 I am sure they were able to optimize the game quite a bit better.
A recent examples of a really good game with a smaller filesize is Just Cause 2. JC2 isn't even 5gb and it has a huge world to explore and packs in a ton of DX11 bells and whistles. Skyrim was also MUCH smaller than people expected in the pre-load and that was a result of Bethesda using better compression this go.
In regards to Max Payne 3, it was so large because of all the pre-rendered shit which takes up huge amounts of space. Also, Rockstar is the WORST at PC ports with GTA IV chewing up more than 20gb and running like ass.
Lately, THQ has done a pretty good job making sure their studios do good PC ports. I would not worry about this too much and just get excited to enjoy what looks to be a fantastic game! Long story short, as your girlfriend says, size isn't everything.