Like all things in audio - it depends on the person - and what they've experienced previously.
My head is not what I'd call overly large - I'm 6ft - so ave to large guy.
I was coming from Beyers & ATs - so my first impression of the HD600 was noticeable and tight clamp. So tight in fact - that I tried to gently expand the headband when I first got them. Big mistake if you don't do it properly ....... CRACK/SNAP.
Anyway - I ordered a complete replacement headband - and this time, extended the extenders and very lightly bent just that part. Pressure relieved slightly. Now they are just pretty snug (compared to Beyers and AKG this time).
So - from my experience - yes they were clampy - but you can alleviate this if you are careful, and over time they do become more comfortable.