I just returned a helmet that fit pretty tight but I thought it was ok. I kept it on while working on the puter. After an hour I started to feel the pressure on my cheeks and ears, it was a Med. Then I read that if, when pushing the helmet in to your face, it should not touch your nose or your chin. It makes sense that in a collision where you get hit in the face, that's going to do some damage. It did touch my nose and chin. So with the pressure on my cheeks and ears and touching my nose and chin, back it went. I am waiting for a Large replacement. I hope this helps.
Richard L.