Joel:

Don't think of these things as capacitors. They are really a version
of an open-ended quarter-wave transmission line stub. The transmission
line section (which has varying impedance along its length due to the
varying width) transforms the open circuit at the big end to an
effective short at the small end.

I believe (though I am not sure) that this shape produces a wider
bandwidth short circuit than does a more traditional constant-width
quarter wave microstrip stub with the same line width as the small end
of the quarter-circle.

A search on "radial stub" is likely to turn up more info (though I
haven't tried it).

73,
Steve VE3SMA