I am on medication now, but when I wasn't I was average weight (5' 3" and 115 lbs). I think it has a lot to do with the hyperactivity of my ADHD - it's hard to gain weight when you're jumping around like a squirrel on crack, even when you much through 1.5 pounds of skittles. I notice it with my cousins too, as they seem to fit into two groups - hyperactive and skinny, or calm and overweight.
Of course, I'm assuming you refer to "average" as in a healthy weight, not "average" as in the actual mathematical average, because most Americans are overweight.
Well mine is more the inattention type, with only occasional hyperactivity. I am getting to be overweight & I seem to be hungry often. Overall I don't eat that often but when I do its huge portions or having snacking episodes.I am over weight---32 jusrt started meds-----