Japan is about 5 times smaller than Indonesia.

Indonesia is approximately 1,904,569 sq km, while Japan is approximately 377,915 sq km, making Japan 19.84% the size of Indonesia. Meanwhile, the population of Indonesia is ~285.7 million people (162.4 million fewer people live in Japan).
Tip: This to-scale comparison of Indonesia vs. Japan uses the Mercator projection, which distorts the size of regions near the poles.
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