A starving Cyrodilic Vampire is almost the equivalent of a Clan Berne vampire. While the Volkihar are even more powerful than the Clan Quarra vampires. The Quarra were the most feared Vampire Clan in Morrowind not only for their sheer brute strength, but also their ruthless brutality. Where a Clan Berne vampire would sneak into a village and feed on someone while they're asleep or abduct them, a Clan Quarra vampire would massacre an entire village and feed on them just because they could.
Also, Skyrim is the birthplace of Vampires in Tamriel, so odds are Skyrim has alot more Vampire Ancients than any other province. Not to mention, one of the concept art pictures that was shown in an interview looked very much like a horrific looking female Vampire. How much you want to bet that's Lamae Beolfag (The Blood Matron, the very first Vampire on Tamriel) in that picture?
Just because the Cyrodilic vampire doesn't go on mass feeding sprees, doesn't mean they could. Remember Hindaril? Who was locked up in Red Water slough because he terrorized Cyrodiil? He couldn't be killed, so they trapped him in there. Tells you that he was strong. Feeding habits have nothing to do with strength. Cyrodilic vampires can be just as powerful as the Quarra if they starved themselves. Their ingame stats even are a bit stronger. They don't go around feeding on people with barbaric abandon because they are more civilized and more intelligent, and Clavicus Vile graced their social skills and steeled their craving for blood, which is why they retain a little more self control. But when starved for decades, they go mad like Lord Lovidicus.
And it's possible that the creature in http://3.bp.blogspot.com/_4lrm97dBqNo/TUFlxQiW7CI/AAAAAAAAAZM/n_xn6TWnSA8/s1600/artofskyrim_013_femmonster.jpg could be Lamae indeed. It's very possible. We have no proof as to the strength of the Volkihar. We just know of their abilities. When the games comes out, if they are in the game, then we can compare all these clans and see which one is stronger. Since Skyrim is the birthplace of vampires, most likely the ones there are stronger. But remember, the Cyrodilic vampires were created before they even entered Cyrodiil. Meaning they probably were in existence elsewhere.