The HD650's are very comfortable after you either break in the headband over time with wearing or if you decide to bend the metal bars yourself. Initially you'll get the similar vice grip from the 280's though.
Bigger heads will feel more squeeze but they are not bad with time. Comfort (lack of) should not be a factor when choosing these headphones. That said, my Denon's are much more comfortable for my head.
Properly adjust your HD650, HD600, HD580. See the link in my sig. This requires bending the metal band in a very specific way before listening to the headphones or stretching them over your head, unless the headphones happen to already fit. I owned the HD280, HD580, HD650.