Data for better educated guess
I have always had to buy a helmet that is too large because I have a wider than usual head. The lidpicker report confirmed this and showed me that almost all helmets are too narrow. I wanted a helmet from Arai, Shoei, or Schubert, but most of them are too narrow. I found from the Revzilla website that the Arai Quantum X helmet is round oval shaped, so I guessed that the XL would do (instead of the XXL or XXXL I usually buy), and it fits quite well.