It's also what you would say after realizing you put on the weight; ie "WOW," due to the fact you put on so much weight from playing.
Joe 2: What is it?
Joe: I've put on a lot of weight, geez.
Joe 2: You bought Cataclysm, right?
Joe: Yeah, so?
Joe 2: WoW weight, gg.
Jim: Man you've got a lot of food. What for?
Jim 2: Just bought Cataclysm, gonna put on some Wow Weight.