Sizing does run small, and much smaller than websites recommend sizing - per measurements we were S and XS heads. After several trades we actually fit in M and L helmets. They are a bit tight when you first put them on, but after an hour or two of your warm head in the helmet it seems to thermal fit a bit and then is very comfortable. The lack of neck pain after a full day of riding is completely worth the premium in the helmet, and I am always hot in all my gear but this helmet breaths well enough for me to wear on warm spring rides, but the wife did not complain about being cold even without a head liner or baclava on some 20F days. Most excellent!