The area of the underground world is no smaller than that of the surface world, not only that, because the underground world actually has three layers. If all three layers are added together, it may be more extensive than the surface world.
However, the area occupied by the belief forces of Haila, the god of death, is very small. It is just in the south west of the Segos mainland, just near the Elven Forest, which is just included in the 500 km range of activities that Eve is now opening to players.
In addition, like most underground city states, Haila's belief forces are also in the first layer of the dark area, that is, the upper layer.
Upper Dark Zone