Heard Island and McDonald Islands is about 4 times bigger than Montserrat.

Montserrat is approximately 102 sq km, while Heard Island and McDonald Islands is approximately 412 sq km, making Heard Island and McDonald Islands 304% larger than Montserrat.
Tip: This to-scale comparison of Montserrat vs. Heard Island and McDonald Islands uses the Mercator projection, which distorts the size of regions near the poles.

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