600 x 800 is the image size, below that is aspect ratio. At 64 x 45 pixels are taller than they are wide so the Blender render appears stretched. If that information isn’t exported then Yaf(a)ray will render with square pixels, so the images don’t match.
From the wiki:-
The size of the rendering in pixels. The actual value is also determined by the percentage buttons (100%, 75%, etc.).
The pixel relationship. The pixels in monitors and video cards are not usually exactly square. These numbers can be used to specify the relative dimensions of a pixel.
Some of the presets use different ratios if you are rendering for T.V.
Like Yafray, Yaf(a)ray doesn’t have access to all of Blender’s functions.
EDIT - the page I linked to is a long time out of date - ‘This page was last modified 14:06, 10 December 2006’
This is more recent - http://wiki.blender.org/index.php/Manual/Output_Formats#Format_Panel