if you imagine the armor gone, the head is actually the right size. It's just steel plate armor is so bulky, and they dont bulk the helmet up as much since it would really strain the neck. So it makes sense.
i would have thought, the steel helmet would make the head look bigger... so the proportions would be pretty much back to normal. I once wore such a thing. It has a thick layer of leather so to not harm the skin. For this reason it is quite a bit larger than the actual head size.
I really like the style, love how large the planes look. After looking at it for a while however the thought: "What is he even guarding in that deserted place anyway?" did come to me, then on the other hand, that might just be me.