They do run smaller than other helmets. I wear a medium in the scorpion Exo 750.
I've been using a Small Shoei X-11 for about 4 years now. I purchased a Bell Star Carbon Cerwinskie last year. When I ordered it they didn't have any Smalls left so I took a chance on an XS knowing the Small & XS share the same shell size. All I would have to do is replace the liner if it was too tight. Turned out to be a good thing I got the XS because I found it to be about the same fit for me.
To be honest part of why I ordered it was because of the Carbon Fiber Construction. I was thinking it would be leaps and bounds lighter than my Small X-11 because of it.
When I pulled it out of the box I noticed how much larger the shell was than my X-11. A quick check on a scale proved that it was lighter but only by a few grams. The regular shell Star would have no doubt been heavier than the X-11.
**The Small Shoei X-12 is no doubt heavier than the Small X-11 because of either the SNELL2010 requirements or extra room to make way for the new emergency pull out cheek pads**
I rode in the Star for a while and was a little disappointed because I read about how stable this thing was supposed to be at higher speeds and on the freeways. Quite frankly I found my X-11 to be more stable. After talking to a couple friends in town who have the same helmet I added the Track Strip thinking it might help. It didn't seem to do much and the thing was still a bit unstable.
I did a back to back test in a mirror with my X-11 and realized the cheek pads on the Star hit my face in a different area. An area where I'm a little lean/narrow and it definitely left a little wiggle room there.
Upon inspection I found my XS Star came with the Firm 50mm pads so I ordered up the Firm 55mm pads, the thickest they make. Success! Now it fits great and is at least as stable as the X-11 if not perhaps a little better.
After living with the Carbon Star for a while I noticed the finer points of the helmet vs. my X-11. It offers a super comfy interior material, it flows more air, and it has a larger field of vision to boot.
All in all not too f'ing bad if you can get it at a lower price point.
My complaints after a season of riding in it:
1. Noise, yea it's a little loud. The helmet will remind you to put your ear plugs in.
2. Chin strap closure system. "If it ain't broke don't fix it". I'd rather have the snap closure like 99% of the other helmets have instead of the magnetic ends. I've had it come unsnapped/unclosed a couple times and it's pretty bothersome when it happens. Something I've never had a snapped helmet do.
3. Chin vent. Holy cow does that sucker need a filter. Having a large bug come blasting through that thing at speed isn't so nice. Most of the time I leave it closed.
I leave the chin curtain in 100% of the time because all kinds of crap seems to get funneled up that way. Again to be fair I mostly use the chin curtain on my X-11 because of this same issue.
Getting something in your eye while riding is not a good idea. I'm 5'10 and still using the stock wind screen though. Perhaps if I installed a double bubble or Corsa it would rectify this.