How funny! My Filas try the same stunt from time to time - until they realize they're much more comfy hogging the whole sofa! Shiva's favorite spot is at the end where she can prop her chin on the arm and watch TV.
I had a St. Bernard years ago and I am feeling your pain! He thought he was a lap dog even after he was well over 150 lbs. but the hardest thing for him to realize is why he could no longer walk under the coffee table!