The author explains the concept of scaling and how scaling affect the images and letter forms. The vector images and letterforms can be enlarged to any size without losing the quality where as bitmap images get pixelized when enlarged. The concept of relativity applies to scaling as well. We expect certain object to be of certain size and scale and it is important to represent the scale ration on the web as well. the main three concepts that make a graphics resemble with the real world is the scale, depth and motion; and the combination of these three will make a graphics look more realistic.
As a user experience designer it is important to have an eye for scales and use them wisely. One of the way to use the concept of scale is to place the objects and scale them with respect to the importance of that object in that particular scenario. The other way is to use the realistic scales related to the other objects around. For example a big flower with a mountain as a background may not fit well in terms of scale. As human relate the things we see on a graphics to the real world it is important to have the right scale related to the other objects around.
Source: http://hcc710.files.wordpress.com/2013/03/scale.pdf