"Sam Raimi and Rob Tapert -- who brought us "Xena: Warrior Princess" -- and Steven S. DeKnight, who brought the world young Superman in "Smallville." Think of this series as "Bigville" -- or at least "Big Budgetville." (Link)
Beyond that, i cant be bothered to search for more articles, my eyes bleed from the "watch Spartacus" links
Where exactly do you have the "official number of 10-15 mill" from ? I've certainly not seen that anywhere.
How popularity is measured i leave to the proffesionals. IMDB is considered the nr1 go to site for reviews and ratings. And IMDB place Spartacus in the current top 5 most popular TV series in the world.
This season for Spartacus opened with 4.1 million viewers, and the illegal torrenting of the show across the world is expected to at least mirror it(Most likely surpassing it) The walking dead,Dexter,Game of thrones, How i meet your mother, Big bang theory and Spartacus are considered some of the most torrented TV-shows in the world. IMDB try to represent the world not only America. The numbers for the season premiere of Spartacus is probably around 10 million which is no doubt one of the reasons IMDB place it in the top.
But you are right in the US Spartacus is not as big as dancing with the stars or NCIS. But across the world TV channels fight for the right to broadcast GoT,TwD and Spartacus.
But as i have said before this is only good as Spartacus hype spreads across the world it will rub a little bit of that hype unto Rome II. It is up to CA how much hype they are willing to accept.