Moscow is about 1.6 times bigger than Johannesburg.

Johannesburg is approximately 1,644 sq km, while Moscow (map shows its 125 districts within the pre-2012 boundary; the area includes New Moscow) is approximately 2,562 sq km, making Moscow 56% larger than Johannesburg.
Tip: This to-scale comparison of Johannesburg vs. Moscow uses the Mercator projection, which distorts the size of regions near the poles.
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