Marchaj's book "Sail Performance" does a good job discussing head shape. The square head does have more area up higher, but b/c of the shape it's much more efficient (more equal loading along the chord length). I don't remember the percentage, but if you look at flow and usable area of a pin head, it looses something like 30% at the top.

I'll pull the book back out if I have time and find the section to reference.