A couple of months ago I purchased a Scorpion helmet in the same size as the HJC CL-14 helmet I purchased when I got my bike. The Scorpion seemed a little tight, but I chose it instead of going up a size, trying to anticipate that the padding might compact a little with wear.
Well, the padding has not compacted, but I am having a hard time going back to the HJC. I'm now used to the Scorpion's tighter fit, but the air flow is much better than the HJC's; the shield will stay in the partially open position even at highway speed; changing from the smoke to the clear screen (for those days when I work laaaaaaaate) is so simple, even I can do it; wind noise is tremendously reduced; and it seems lots lighter.
The kicker, the HJC, because there is so much space between my cheeks and the helment, seems less safe. (Don't get me wrong, it still passes all the helmet-fitting tests. It just doesn't hug my cheeks and I've grown accustomed to that with the Scorpion.)
I just wanted to say something about this because I have talked with some guys and read about others who feel the need to bump up a size with Scorpion. I wanted to say that if you can get used to your head being "hugged" then bumping up might not be necessary. Your mileage may vary.