The reason is that the Mercator projection (the ones of the common world maps) distort the sizes of the countries. That projection is great for navigation, but doesn't show the world like it is.
This is from wiki:
The Mercator projection
is a cylindrical map projection
presented by the [ame="http://en.wikipedia.org/wiki/Flemish_people"]Flemish
[/ame] geographer and cartographer [ame="http://en.wikipedia.org/wiki/Gerardus_Mercator"]Gerardus Mercator
[/ame], in 1569. It became the standard map projection for nautical purposes because of its ability to represent lines of constant course
, known as [ame="http://en.wikipedia.org/wiki/Rhumb_line"]rhumb lines
[/ame] or [ame="http://en.wikipedia.org/wiki/Rhumb_line"]loxodromes
[/ame], as straight segments. While the linear scale is constant in all directions around any point, thus preserving the angles and the shapes of small objects (which makes the projection [ame="http://en.wikipedia.org/wiki/Conformal_projection"]conformal
[/ame]), the Mercator projection distorts the size and shape of large objects, as the scale increases from the Equator to the poles, where it becomes infinite.
[ame="http://en.wikipedia.org/wiki/Mercator_projection"]Mercator projection - Wikipedia, the free encyclopedia[/ame]