Yep they can get that tall and taller the Russian giant gets to around 10 ft and they is some that will grow even bigger have a look at the link the one you have grown looks fab though

http://www.ebay.co.uk/itm/TRIFFID-SUNFLOWER-COLLECTION-3-ENORMOUS-SUMMER-MONSTER-VARIETIES-IN-ONE-DEAL-/390646841048?pt=UK_HomeGarden_Garden_PlantsSeedsBulbs_JN&hash=item5af45dbed8

Looks as though its enjoying the shelter of that lovely wall.

I used to grow a few dozen sunflower seeds in polystyrene cups, give one each to all the neighbours kids and award prizes for the tallest, the biggest flower and the best specimen. It got so seriously competitive and people used to resort to underhand tactics and sabotage so I had to stop. Kids being kids you might think? No, it was their parents.