I’m guessing you’re talking about this website: https://thetruesize.com?
So, the Earth is a sphere. It’s a mathematical fact, that if you [map the surface of a sphere onto a plane, you’ll get distortions](https://en.wikipedia.org/wiki/Map_projection). Shapes, sizes, directions, distances, you can preserve some of those properties, but not all of them.
The Mercator projection is one of the most common map projections. It’s a useful projection for navigating a boat, but it makes things near the poles look much larger than things near the equator.
What that website does is let you drag a country around the globe, and shows you how the Mercator projection distorts it at that location. So, if you drag Greenland closer to the equator, it appears to get smaller, because the distortion is less. This lets you compare the sizes of different places, because you can distort them both in the same way.
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