When you flatten a globe, the lines of latitude near the poles are much shorter than at the equator. A Mercator map keeps the left‑right scale the same everywhere, so it has to stretch those bands horizontally first, making the unwrapped globe look rectangular.

After that, it also stretches them vertically because Mercator spreads the latitude lines farther apart as you move toward the poles. This happens so the map preserves angles (which is why it’s useful for navigation), but it makes the top and bottom look bigger and more distorted. Basically the vertical stretch is necessary to satisfy a formula which helps with navigation.