I'm generally impressed with the casting. In the case of Gregor Clegane, it was probably quite the chore, and to get an actual match would have been impossible if not even worth it.
The actor playing "The Mountain That Rides" is 7' 5" tall, when the character is actually about 7' 11" tall and some 420 lbs. I imagine they chose this guy for his build, looks, and ability to play the character without coming across as a flake.
And, you know, the camera adds pounds. And filming techniques can make people look taller. They only need to account for a missing half foot in height.
Still, 7' 5" is impressive. To give you an idea, below is a reference: