It was more likely his age - they say he is old - than his weight being the problem with the stairs. I'd not be surprised if he has a touch of arthritis, and hip problems are also common in the giant breeds.

Our first Saint Bernard once got stuck at the top of some stairs - she was about a year old, so not yet full grown, and the stairs were so steep the loose hide (that she eventually grew into) rolled down to cover her eyes whenever she started down.

The only way his size factored into it, I am sure, is that his owner couldn't simply pick him up and carry him back up the stairs! Poor Arnold, at least his owner will know not to let him down those stairs again!