South Georgia and South Sandwich Islands is about 5 times bigger than Tonga.

Tonga is approximately 747 sq km, while South Georgia and South Sandwich Islands is approximately 3,903 sq km, making South Georgia and South Sandwich Islands 422% larger than Tonga.
Tip: This to-scale comparison of Tonga 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. Tonga