That site only seems lock the zoom value of the two maps together, not correct for distortions. E.g. zoom in on Svalbard on one side and the Democratic Republic of the Congo on the other. Svalbard appears larger despite being many times smaller. This means if you zoom into Longyearbyen it will appear several times larger than it should compared to say Kinshasa.

Longyearbyen is a pathological example but it's quite easy to end up thinking a city in the UK is ~1.75 linearly and ~3x by area compared to one on the equator using this site.