The farther away an object is from the human eye, the smaller it appears. It may seem even smaller if an object in the foreground looks larger, because of the relationship between those two objects.
Perspective also can affect the appearance of straight lines. Any lines in an image will appear to converge the farther away from the viewer's eye they are or as they approach the horizon in the distance.
Eye level also determines what a viewer is able to see in a photograph. The first photo below was taken from a standing position, and the second, from a seated position. Notice how the lines appear to converge (or not), and objects seem smaller or larger depending on their relationship to the rest of the scene.