Expanded world??

The project contains a world partition, where the landscape is imported from a Heightmap, and scaled up to cover an area of about 9 x 9 km, divided in 4 regions. But the world partition map shows this as the left side of a world that is about 10 times wider:

Does this impact memory usage and overall performance, and if so, how can I scale this back to the actual size of my world?