This is a screenshot from Google Maps, at the highest possible scale. Most of us grew up around maps similar to this one, and this is how we picture the countries on Earth.
In order to represent a spherical object on a plane, maps have to integrate a certain degree of distortion. There are several standard ways of projecting maps. One of the most commonly used is the Mercator projection; the projection method used by Google in the map above.
Just how bad the distortion is is difficult to conceive. In fact, for a long time, it seemed irrefutable to me that Greenland was one of the most impressive pieces of land on the planet. Does’t it look bigger than South America?
Well, it’s not. Consider the following pairs:
1) Democratic Republic of Congo (in pink on this map) and Greenland.
2) Alaska and Mexico.
3) Iceland and France
The Democratic Republic of the Congo (2,267,048km2) is in fact slightly bigger than Greenland (2,166,086 km2). They are respectively the 11th and the 12th largest countries in the world.
Mexico (1,972,550 km2) is larger than Alaska (1,717,854 km2).
France is 5 times bigger than Iceland.
The Mercator projection stretches the map at the poles in order for the longitude lines to appear parallel on paper. This makes the countries closer to the poles seem much bigger than they actually are. The error at the poles is virtually infinite.
Google is, of course, aware of this. If you slide your map up and down, you will notice that the scale (at the bottom left) adjusts accordingly. The scale is only valid for the latitude on which it is positioned.
At city city level, however - where Google maps are the most useful - the Mercator projection is very accurate.
You can visit the USGS site for a complete review of the different map projections and their properties.