Most manufacturers make 2 (sometimes 3) sizes of outer shells. They adjust the ammount of foam inside to change the fitting size.
Shell size varies from manufacturer to manufacture. Shoei's for example used to be huge, but not so with their latest RF1000.
When picking a helmet, find the smallest helmet that you can put on. If the cheekpads make you look like a chipmunk, dont worry about it, extra $15 and you can get different ones.
The reason is multiplefold. A tighter helmet will obviously protect you better. And helmets break in, as in the get larger on the inside over the first couple of months. The looser the helmet, the more windnoise it will generate, and the more buffeting it will exhibit at speed.
Wind noise is the no. 1 couse of getting tired on long rides, and buffeting is incredibly annoying.