Japan is clearly the larger of the two: about 377,915 km² against 244,376 km², roughly 1.5× at true size. Drag one onto the other on the map above to feel the gap.
Japan sits in Asia, United Kingdom in Europe. On the old Mercator map the latitude gap distorted how large each felt beside the other; at true size the answer is simply 1.5×.
The old map muddied it further: Mercator inflates Japan about 1.6× and United Kingdom about 2.9×, so on a Mercator-style map this pair appears 1.2× apart instead of the true 1.5×.
United Kingdom fits 1× into Japan.
Japan vs United Kingdom: key figures
| Measure | Japan | United Kingdom |
|---|
| True area | 377,915 km² · #63 | 244,376 km² · #80 |
| Mercator inflation | 1.6× | 2.9× |
| Apparent on Mercator | 599,789 km² | 707,713 km² |