Sydney is around the same size as Melbourne.

Melbourne (the 27 councils of metropolitan Melbourne, without the Yarra Ranges, Cardinia, Mornington Peninsula and Nillumbik) is approximately 3,915 sq km, while Sydney (the 30 councils of metropolitan Sydney, without the Blue Mountains, Hawkesbury, Wollondilly and the Central Coast) is approximately 3,810 sq km, making Sydney 97.31% the size of Melbourne.
Tip: This to-scale comparison of Melbourne vs. Sydney uses the Mercator projection, which distorts the size of regions near the poles.
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