I noticed that the image full-image.php returns is compressed. Reading up on imagejpeg on http://us2.php.net/imagejpeg tells us that without quality specified, it will use the IJG default (about 75).
I would suggest changing the (nearly) last line of full-image.php from
imagejpeg($newim);
to either
imagejpeg($newim,NULL,100);
or
imagejpeg($newim,NULL,$quality);
I would lean more towards defaulting to 100, or perhaps adding a full sized image quality option to the admin options and using $full_quality or something like that.
Found in version 1.1.3
-Thomas