No, it is not huge, though it's priced as if it is. Skyrim is hundreds of hours worth of gameplay. Dawnguard costs 1/3 of the original price tag and we do not get 1/3 of its gameplay hours, not even close.
I think you are approaching this like a linear game, where you pay $X and go on a mission to some dungeon and perhaps get a new companion and that's it.
This DLC will enhance the gameplay in the rest of the world, which is something you can't exactly measure in terms of hours.
Let's say a typical playthrough of Skyrim lasts somewhere around 100-200 hours for most people. For many of us, this DLC will make it really enjoyable to do two completely new playthroughs, one siding with the Dawnguard and another siding with the Volkihar. That is a great value IMHO, and I imagine we are going to enjoy playing a werewolf or vampire and leveling all the perks and abilities you get over the course of a complete playthrough.