In this post, I want to focus on how to make proportional circles using the KML Icon element. This is the easiest and most efficient way to make proportional symbols.
This map shows the population in each country of the world in 2005 (source: UNdata). See this blog post for information about symbol placement and how they are mathematically scaled.
A KML icon has to be an image. I hope vector support will be added in the next major KML version. A circle can be drawn using one line of SVG, and I think KML should support such basic shapes. Still, KML has powerful ways of styling icon images, and I've used these elements to create proportional symbols.
I've made one icon for the proportional symbols map. The icon is a white circle on a transparent background saved as a PNG file (200 x 200 px). The icon size has to be bigger than "normal" icons since it will be scaled proportional to an attribute value. The icons are colourised and scaled using IconStyle elements. Only one image is thereby needed to make circle symbols in different colours and sizes. The KMZ file containing KML and the image is only 19 kB.
The drawback is that the map is only viewable in Google Earth (so far). This is how it looks in Google Maps and Microsoft Live Maps (Virtual Earth). The icons are not colourised or scaled.