On Windows 3.1, the borders of a button are always 3px thick, so the corners are always the same size no matter what size the button is.
On System 6, buttons have rounded corners, where the radius is calculated from the button size. So a very small button might be drawn with a 1px or 2px corner radius, but a very large button might have a 50px corner radius.
With Decker's font metrics, I'm generating 20px buttons, so I changed the corners to reflect the border-radius used by the original System 6 buttons at that size.
