Maybe they're talking by population. Cities are ranked by population. New York is a large city. Means it has a lot of residents. Generally. Were you to be looking to find someone or something Europe might very well be a large continent.
The accusative has to represent the object of something (either some action or a preposition). But there's no action going on here. (Europe isn't doing something to a big continent. Being is not doing.) So you default to the nominative.