Hmmm, I wonder if your body is really tense while you're knitting. I'm kinda thinking that a scarf shouldn't be too heavy to knit. Have you tried changing your position every once in awhile? Are you holding the needles up high in front of your face rather than in a neutral position?
What I'm getting at is that maybe the yarn isn't really heavy, you might be doing something to cause strain and that makes it feel heavy.
In my very best Austrian doctor voice:
Ve must do somezing to help you relax. Tell me about zis compulsion to make knots in shtring...