The xen has a few different straps inside to adjust the helmet size both ways, so you can adjust it totally to fit your head. Just stick a tape measure round your head, then get the nearest size, then your will be able to adjust it to suit.
On the xen you can adjsut, the neck strap, the fit around your head, and finally how high or low it sits on your head.
You cant to as much with the semi mx, its really hope that it fits, and all you can adjust is the neck strap.