Nearly a tie: Japan and United Kingdom differ by barely -35%% in area — about 370,074 km² km² against 239,616 km² km².
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 0.6×.
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 0.6×.
Japan fits 0× into United Kingdom.