If you weight it by the actual size of territories rather than just the extreme boundaries then the center is indeed in Spain, because the overseas regions are

If you weight it by the actual size of territories rather than just the extreme boundaries then the center is indeed in Spain, because the overseas regions are still rather small in comparison to the European part of France. If you use only the extreme boundaries then it gets a lot more interesting and I would say that the center is closer to the center of the Earth than to its surface.