Maybe the quality operation does not influence the size of the resulting pdf? Will you get the 2.53MB size when you omit the 'diff_image.Format = MagickFormat.Png' statement?
Does your resulting pdf from the command line has a size of only 260kB? Are you sure both pdf's are created from images with the same dimmension and format?
The quality operator for png does not work the same as for jpeg images. It is used to set the compression level of the image. You can read the following post to see how it works: http://www.imagemagick.org/discourse-server/viewtopic.php?f=2&t=24134#p103040
Does your resulting pdf from the command line has a size of only 260kB? Are you sure both pdf's are created from images with the same dimmension and format?
The quality operator for png does not work the same as for jpeg images. It is used to set the compression level of the image. You can read the following post to see how it works: http://www.imagemagick.org/discourse-server/viewtopic.php?f=2&t=24134#p103040