I've also found that fit varies wildly and unpredictably with sizing.
In other words, the magazines and review sites all test the size M or L. Those of us with XL or XXL (or beyond) noggins are pretty much on our own. The "narrow" list at Webbikeworld turns into a work of speculative fiction when you add an X or two to the size.
The Icon Airmada, for example, is a very damn nice helmet, and is supposed to have a long-oval shape. However, the larger sizes (XXL and XXXL) don't have the same shape and aren't even much bigger. (I'm usually an Xl or XXL, and I could barely pull the XXXL Airmada onto my head, and the crown shape was the same ol' perfect circle anyway...) Same with Nexx, Scorpion, and many others. HJC changed to a much rounder shape a few years back, so they're dead to me...
Arai seems to be the only helmet maker that offers consistency in fit and a true long oval in all sizes. However, most of us don't have $600 - $800 to spend on a freaking helmet.
I'm currently wearing an XXL Shoei Qwest -- it's not perfect, but it's very, very good.
The only lesson in all of this is perhaps that you have to go try them on in person. Support your local moto shop.