There are a lot of tricks the Arch plays on our eyes. For example, most people find it hard to believe that it is exactly as wide as it it tall. In this photo, it looks like the family taking pictures gives us a sense of scale. But the Arch is so tall and, viewed from the side, it recedes more and more sharply as it goes up, that we really can't gague what we're looking at. One of its many wiles.
By the way, there is another new Arch photo today on my St. Louis Daily Photo Blog.