[metapost] How to use a thinner pen?

[Troy Henderson]

It's very likely your viewer.  That is, GV (and others) eventually
rasterize images as a last step to displaying them on the screen.  As
such, they probably have a minimum stroke width that is allowed and
anything smaller than that gets rounded up to this minimum amount.
I'm seeing something like this in Acroread.  As I zoom in, the size of
the stroke doesn't change but of course the rest of the graphic does
(magnifies).  So, the relative sizes is changed meaning that the small
stroke is more accurate when magnified.

Troy