El Salvador is about 18 times smaller than Japan.

Japan is approximately 377,915 sq km, while El Salvador is approximately 21,041 sq km, making El Salvador 5.57% the size of Japan. Meanwhile, the population of Japan is ~124.2 million people (117.6 million fewer people live in El Salvador).
Tip: This to-scale comparison of Japan vs. El Salvador uses the Mercator projection, which distorts the size of regions near the poles.