I suddenly realized something.
Have you noticed how in most JRPGs when you travel on the overworld, if you traverse out of the world map from the left edge you appear on the right edge, and likewise if you traverse out of the upper edge you appear on the lower edge? Thus the world is just one contiguous surface, like Earth, really. If you don't think too much about it, it makes perfect sense and is logical.
But stop a minute to actually think about it. Consider Earth's world map, eg:
If you traverse out of the left edge, you just continue on the right edge. They are connected. So this is ok.
However, what happens when you traverse out of the upper edge? Certainly you don't suddenly appear on the lower edge! The upper and lower edges are
not connected! (Instead, you appear on the upper edge again, just on a different place.)
So how exactly does this work with JRPG world maps? You can't connect both edges like it was a contiguous surface like that. At least not if you are trying to depict the spherical surface of a planet.
The only possible explanation is that the planet has actually the shape of a torus. The only way a rectangular area can have both opposing edges connected is if it represents the surface of a toroidal shape.
Thus the planets in most JRPGs are toroidal. Of course this raises the question how such a planet can possibly form.