Your're not Rob Ford by any chance, are you? :-)
I have a medium to large head and I need the next to smallest setting, just for comparison. Is it the clamping or lateral width that is the factor here? They are the teeniest bit clampy, but less than several other phones, and maybe they can be bent a little (I have not tried it,
but most phones I have found can be adjusted this way).
I actually found the HD600 too tight and clampy and more uncomfortable.
Maybe Hifiman could chime in with suggestions for adjustment...
If you want comfort for a big head, I find the big Grados very accommodating (and I really like the GS1000 and there is a new 'e' version out now).
LOL, Rob Ford has really penetrated into American pop culture when he gets referenced on a head-fi thread!