i would say it's his mask as i recall without his mask it looked better in your old chapters. the mask thing i would say is the most logical explanation do you usually start drawing the mask first? or do you draw the head first?
I draw body shapes first...ovals and such. I never do any detailing until after I have the pose drawn the way I want it to (and I always thought I drew Ulquiorra's head!bubble the same size as Orihime's, which further frustrates me when it looks bigger in the end.) It's why I could never understand why Orihime's head always looked impacted...my ovals haven't changed...I guess I just drew her hair too poofy at the front and it made the back look shorter.