I guess we'll have to consider the possibility of alot of new content added to skyrim in general, which would make up for solstheims potentially small size (:

It has been in development since before or around the same time as Dawnguard, which has been nearly 6 months (assuming the release is december), and shivering isles was released 7 months after oblivion so in comparison they should have the same amount of content if not more.

Think of the Forgotten Vale. Pretty decently sized world space for a little blip on the edge of NE Skyrim.