I don't know which is the lightest one. The Arai website doesn't show the weights of the various models?
Arais are definitely NOT the lightest helmets though. I think that they are pretty much in the middle when it comes to weight.
I haven't done a hard search but I have yet to see any other helmet makers offering different models to accomodate different head shapes.
With the other makers, it's luck that you'd find a helmet that fits you, and then hopefully that helmet is from a quality manufacturer. With Arai's entire line of helmets dedicated to different head shapes, you will definitely find one that fits you. But the cheapest Arai with non-removable liners and less ventilations will set you back in the mid to high $400. Even the Arai offroad helmets are knocking at $400 or more. But then what price is your head?