Fascinating. As I was reading the snippet, a combo of the first two was my guess: because the hair or forehead are empty space, people tend to ignore them and fill the space available with the features the notice. I'll bet if people draw the outline of cities on a blank map of a country or state they would over-estimate the size of the cities. The part where the hairlines are drawn in the correct place goes against my "hair as ignorable space" hypothesis, though.