Part of that I think is because his cape gets pulled behind his shoulder.
We are so used to seeing Vader as a big dark triangle that it can be a bit of a shock to see his neck and shoulders without the cape, it changes a nice stable triangle into a tall oblong.
I never liked that bit when I was a kid because I had it in my mind that he was such a powerful being that he would have total control over his clothing (see now the full power of the darkside, no creases or fluffballs).
Now I rather like it because it seems natural.