The middle circles shown in Figure 6-24 are the same size, although they definitely don't seem so. I know it for sure because I drew them myself; they are both 80 pixels in diameter. But “80 pixels” might not mean anything to you. It's just a number. We can only make sense of this number in comparison.
If you're surrounded by giants, you will seem small. If you're surrounded by pygmies, you will seem tall. We can try to be “objective” and not surround ourselves with anything or anybody, but then people have a hard time coming to any judgment at all. We can surround ourselves with giants, pygmies, and tons of average-sized people, but then we run into the risk of overloading the audience. So, where's the fine line?