I tried on the Scorpian 400, 700 and 1000. The small 700 and 1000 series fit and I had to go to a medium for the 400 but they really didn't feel that comfy (well, at first try). I also tried on the Shoe RF1000 and the small fit ok...but felt a bit short where the medium fit better length wise but was way to big around.
Scorpions were my first helmets ... I had both 400 and 700. There is a noticible difference between the 2 ... the 700 is lighter and doesn't have the same lift problem that I had with the 400. I felt like I had to fight to keep my head attached to my neck from the lift on the 400. However, I found that, after a time, the Scorpion 700 didn't fit as comfortably as I would like.
The helmet I am attracted to is the Arai profile flag helmet. Ok, well I like the british flag on it since I ride a Triumph, but more than the graphics, I have heard many good things about it. Unfortunately, There isn't one in El Paso for me to try on...and I hate messing with mail order retuns..
I can't say how the Icon and HJC compare with the Arai. But ... to continue the saga that I started above ... since that Scorpion didn't fit as well as I would have liked, I decided that I should get a different helmet. So I went off to Motorcycles Unlimited here in Houston (sorry Nathan) with my heart set on a Shoei. (I had read a lot of reviews online and decided that's what I wanted.) Patrick had me sit down and then his wife (I can't remember her name right now) measured my head ... around, side to side, front to back. It turned out that, apparently, I had a "long oval" head and that's why the Scorpions didn't fit as well as I felt they should. While they would have sold me a Shoei if I really wanted it, they strongly recommended that I look at the Arai Profile. It was, without question, the best helmet purchase that I think I could have made. I only had that particular helmet for about 1 1/2 year (dropped it one too many times) but it was comfortable and snug, fit great, didn't 'lift' my head and was pretty light as well. So, when I had to retire that helmet from dropping it, I got myself another Arai Profile. I really, really, really like that helmet.
I know that I can't go into the details that Nathan does ... but, if you have the head shape that I have, then the Profile might be a good choice for you. And Arai is, AFAIK, the only helmet manufacturer that goes into detail with the helmet shapes. You can get more info on that here:
http://software.mogistic.com/araipub/default.aspx?pageid=132. There's also a video on how to fit a helmet.