El Salvador is about 1.3 times smaller than Burundi.

Burundi is approximately 27,830 sq km, while El Salvador is approximately 21,041 sq km, making El Salvador 75.61% the size of Burundi. Meanwhile, the population of Burundi is ~12.7 million people (6.1 million fewer people live in El Salvador).
Tip: This to-scale comparison of Burundi vs. El Salvador uses the Mercator projection, which distorts the size of regions near the poles.