The Mercator projection stretches and distorts landmasses that are far from the equator. This is due to the 'flattened' style of the map. The farther from the equator, the more pronounced the effect.
The Robinson projection attempts to compensate for this flaw by maintaining the 'curve' of the longitudinal lines. It still suffers from exaggeration near the north and south poles, but to a lesser extent than that of the Mercator projection.