I'm not sure if his logic is right, but helmet sizing is a weird thing.
For example, I usually have Medium (57) helmet in HJC or AGV. But there is no way on gods green earth that I can get a medium Shark or Shoei on.
Below is part of an article on 'Choosing a Helmet'
FIT
This is the biggie. Assorted folk around the trade and riding community have distilled their wisdom into the following:
Try as many brands as possible - they're all different and only a couple will really suit your head
It must be tight - if you can smile without looking like a disturbed puffer fish, it's too loose
It shouldn't hurt - wear your selection for 10 minutes (read a newspaper) - if it's starting to hurt in some places, it's the wrong shape;
Can you work the chin-strap and visor easier on this model than another?
Never mind the paint, check the quality - how does the finish of the upholstery or visor compare to another helmet of equal value?
The whole article can be found @
http://www.bikepoint.com.au/portal/alias__bikepointau/tabID__5766/BikeArticleID__117374/BikeTipType__Gear/DesktopDefault.aspx