Stupid ass washing machine...
I thought it looked a bit weird at first, and I couldn't understand why I felt that.
But the reason it looks "skinny" - and you're right, it does - is because in Halo: CE all Spartans had the two visor wedges shadowing the visor, making it look smaller. For some reason the helmet in the two shots we see in the trailer doesn't have the shadows, thus the skinny feel.