In LilyPond 2.5.27, barlines look far thicker than in previous
versions, and far thicker than is visually appealing. This seems to
be a byproduct of the fix for inconsistent width of barline
connections between staves.
Specifically, I'm seeing this on Mac OS X version 10.4.1 in Apple
Preview or other apps based on Apple's PDFKit framework (e.g.,
LilyPad in progress).