Melbourne is about 1.7 times smaller than Ho Chi Minh City.

Ho Chi Minh City is approximately 6,773 sq km, while Melbourne (the 27 councils of metropolitan Melbourne, without the Yarra Ranges, Cardinia, Mornington Peninsula and Nillumbik) is approximately 3,915 sq km, making Melbourne 57.81% the size of Ho Chi Minh City.
Tip: This to-scale comparison of Ho Chi Minh City vs. Melbourne uses the Mercator projection, which distorts the size of regions near the poles.
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