just as a note, depending on the style you have, they make the lining and cheek pads in different sizes, so you might be able to move up or down depending on your head size.
i had an XL that was a tad too large when i had lots of hair- changed the liner and cheek pads and it worked...cut my hair and then i needed to go a size down w/ the stock size. A pal of mine had the opposite problem- he likes his helmets tight but got smaller lining and cheek pads to give him the right fit.
you can find the lining and pads on ebay on sale now and then, but pay attention to how Arai sets things up because if you dont understand how their lining and pads fit, you might purchase the wrong size...