As a firefighter/EMT I see the quality and fit of a helmet as being critical. Most helmets are made on a predominantly round mold and there variations between companies. These may not properly fit all head types, to find a helmet that fits your head properly it is necessary to try it on for fit, there are a number of good descriptions of methods for properly fitting the helmets on the web.
If you go through the process of purchasing a helmet over the net make sure of their return policy and take into account the cost of return shipping and time and paperwork it may take to get a helmet that will serve you well. There are a few companies that make helmets that accommodate heads other than the average such as Arai, though they are at the high end of the price spectrum. Of course, it's your head.
08 Flat Black Wee - Calsci windshield, OEM Knuckle guards, OEM Center stand, SW Motech crash bars & skid plate, GIVI E46NJ topbox, GIVI E36NJ side cases, GIVI tubular racks, AMOTOSTUFF tailbox mounting plate, Wolfman Denali Tank Bag, Wolfman Tank Paniers, custom leather seat, Dash shelf, Thermometer/voltmeter, Grip puppies, Superbrace