graphicsmagick is doubling the size of “some” PNG files

Question

Using GraphicsMagick to read in a PNG file and write it out under a different name is causing the file size to increase by more than double. This only happens to some PNG files and I can't figure out what is causing it.

Example problem PNG: http://www.google.co.in/logos/2011/verne-hp-3.png

I have tried everything from disabling the opacity channel to using different compression types to changing the interlacing type but nothing is helping me understand what is causing this problem.

If anyone has any suggestions on what might be causing this, your help would be very welcome.

Thank you.