In proportional maps, symbols of different sizes represent data associated with different areas or locations within the map. As an example, the countries' capitals can be represented with a circle proportional to their population:
This map contains a representation of European countries. They are drawn all using the same symbology. The information is pointed out by the circles, a nongeographical feature, with a radius proportional to the residents. For the reader convenience there are also some labels, but he may also guess the name of the capital from its position.