Maybe you are confused with The Hague, which is smaller than both, but the seat of the parliament and government? (Or perhaps you are looking at the surface area, which is not really interesting in this context.)
Well whaddya know. I was actually thinking of the city agglomerate, FAFAIK the metropolitan area of Rotterdam breached 1M population before Amsterdam did. But I really haven't kept up with the last ten years of city developments.