I'm not sure on Shoei's but most helmet makers have two shells per helmet model. Usually XS, S, M.....use the small shell and L, XL, XXL use the large shell. THe only real difference in the size is the cheek pads. Buy a size smaller than what you currently have. This should snug it up a bit and if it lifts off your head will be in it! I ran into the same problem with my AGV X-vent, except mine was too tight so I had to get a size larger pads. Hope this helps! But before you order anything i would email shoei and find out what helmets share the shell.