Skyrim: Dawnguard

[Sebiale]

Well it seems to be considered a plug-in, not an expansion like Shivering Isles.

Also,

However, as far as I can understand, we're getting a single new quest line that takes place mostly on the old map and partly in a demon realm that is akin to Oblivion. No new cities. No massive swaths of land. Basically, not the caliber of expansion that one would expect after the sheer time that Bethesda has spent making this.

Where you get this info?