South Georgia and South Sandwich Islands is about 401 times smaller than Mongolia.

Mongolia is approximately 1,564,116 sq km, while South Georgia and South Sandwich Islands is approximately 3,903 sq km, making South Georgia and South Sandwich Islands 0.25% the size of Mongolia.
Tip: This to-scale comparison of Mongolia vs. South Georgia and South Sandwich Islands uses the Mercator projection, which distorts the size of regions near the poles.
Compare further: South Georgia and South Sandwich Islands vs. Mongolia