Last time i checked, my BMI was at 18.5. So yes, i am, as a matter of fact, a skinny guy. Which might also tell you something about how hard i fell on my ass to make cracks in that ice. =P
I also happen to have a large skull. I had to use a size XXL crash helmet when driving go-kart. Weirdly enough though, my head doesn't seem any bigger than usual. >.> (Now imagine what hell of a headbutt i can give you...)