The Transalp Helmet only weighs 390 grams, which is hundreds of grams lighter than a regular certified helmet. Plus it's very durable...check out the video below of the guy pounding the polycarbonate helmet into concrete! Not a scratch or dent!?! It is EN 966 certified, I believe the lightest one on the market, at least that I can find.
The outer shell seems to be one size fits all, and the inside padding is then fitted to size for your head. Make sure to do your measurements correctly when ordering a helmet, in centimeters as this is an Italian company. At €120, it's a great price, especially with the euro exchange falling right now.
The helmet seems to fit pretty well, and there is a wheel on the back (much like a bike helmet) that can be used to cinch down the fit so it stays where you want it. Bottom line, it's light and that's less I have to carry up a lot of hills during the race!