Rotterdam is about 8 times smaller than Moscow.

Moscow (map shows its 125 districts within the pre-2012 boundary; the area includes New Moscow) is approximately 2,562 sq km, while Rotterdam (map shows its 22 named neighbourhoods; the area covers the whole city) is approximately 319 sq km, making Rotterdam 12.47% the size of Moscow.
Tip: This to-scale comparison of Moscow vs. Rotterdam uses the Mercator projection, which distorts the size of regions near the poles.
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