Bingowings said:
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.
I doesn't add up - all the other shots of Vader walking through Echo Base and promotional photography shows his helmet being longer. Prowse is looking vertically straight ahead after he passes through the girders with no tilts as he's walking (short), and then as Vader is walking down the corridor, longer helmet.