GISS is fairly self-explanatory, but the gist is that the image thumbnails in GIS results get a thick borderBottom. The yellower the border, the larger the linked image is.
Each results page contains so many images (a good thing) that there's likely to be at least one very large one that is so large in comparison to most of the rest of the images on the page that doing the color coding proportionally (i.e. giving the thumbnail of an image that was 50x larger than the next-largest image a 50x yellower border) made GISS nearly useless. Instead, the linked-images' areas are calculated and ranked. The largest image on the page gets the yellowest borderBottom but the thumbnail of the next-largest image will be just slightly less yellow, even if there's a large difference in their areas.