Gamasutra has an interview with Pete Hines, product manager for Fallout 3, about Bethesda's philosophy for DLC, and how it's changed over the years. Quoting: "All these people are out there playing our game by the hundreds of thousands on a daily basis and we want to be able to bring those folks something they could do in a much shorter time frame, rather than just saying, 'See you next year.' That instantly ruled out doing a big expansion because those things just take so damn long to do. So we started looking at the biggest stuff we'd done that people really liked, but that we could do in smaller, digestible chunks. That's where we came to the Knights of the Nine model — it's substantive and it adds multiple hours of game play and new items, but we can do it in a time frame that allows us to get it out without waiting forever. That's what we've gone for with Fallout 3."