That's true, but in terms of actual size it's just not as big. Someone made a map where they measured the time it took to run from the largest horizontal points across the two areas and the two widest North <-> South points and Northrend is enormous.
Totally feels big, but in terms of actual size Northrend is the biggest we've gotten so far.
EDIT: Found it - just adding the link since it's a bit big.
http://i.imgur.com/5u3wHHu.jpg
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I remember the immediate feeling of..."uh oh" that I felt after a few hours of 100. It was not a happy feeling.