A figure whose only real subject is size.
Scale is the entire brief. Nothing in the anatomy tells you how big he is supposed to be, so the helmet and the proportions have to carry it alone.
Big is not a shape. You cannot sculpt big, you can only sculpt the things that imply it, which mostly means keeping the detail coarse and letting the large forms stay large.
The helmet, almost entirely. Everything below the collar is there to make the helmet look inevitable.