District of Columbia is about 686 times smaller than Mississippi.

Mississippi is approximately 121,489 sq km, while District of Columbia is approximately 177 sq km, making District of Columbia 0.15% the size of Mississippi. Meanwhile, the population of Mississippi is ~3.0 million people (2.4 million fewer people live in District of Columbia).
Tip: This to-scale comparison of Mississippi vs. District of Columbia uses the Mercator projection, which distorts the size of regions near the poles.