Map-making has always had the problem of trying to fit the surface of a (roughly) ball-shaped object on a flat piece of paper.
A map reflective of the true proportions of our planet would look something like this:
However, most traditional world maps look like the following. It has the map (and thus the depicted land masses) stretched to fit the rectangular shape.
This in effect enlarges or over-represents the land masses that are farthest away from the equator. Because the continents in the north are closer to the North Pole than the continents in the South are to the South Pole, this exaggerates the size (and thus the implied might and importance) of the Northern hemisphere in contrast with the countries close to the equator and in the Southern hemisphere.