Helmets and heads are kind of hard to match up without some adjustments. This year, I retired my "L - Large" Nolan N-102 (couldn't buy replacement liner to rebuild it again). Bought a "L" Scorpion EXO AT950. Nope, too tight. Exchanged for a "XL" and it seemed to work.
Locally, I rode 4 days in a row during a warm spell in February (2017) and started to think I had a sinus infection from cool-weather riding. Pain around the left eye / sinus area. Rode one more afternoon and the pain grew to include my scalp (only left side) to the point my hair hurt to touch, constantly, day after day.
Dr. said it was trigeminal neuralgia (aggravated nerve pain) I must have gotten from the helmet being too tight on a "square" head. No medicine took pain away but faded over 2 month period.
I bought a "2XL" EXO AT950 and have been lightly sanding it here and there to get rid of any "hot spots" mainly across the forehead (sharp edge of the hard foam). No more trigeminal neuralgia for me.
For me anyway, it is hard to get a proper fitting helmet right off the shelf.