The Underworld's area wasn't any smaller than the surface world. In fact, because the Underworld actually had three layers, if you added them all together, it might even be more expansive than the surface.
However, the area occupied by the faith forces of Death God Hela was very small, just in the southwestern part of the Segis Continent, right near the Elf Forest. With the 500-kilometer activity range currently allowed to players by the Goddess, it just happened to be included.
Additionally, like most underground city-states, Hela's faith forces were also located in the first layer of the dark territory, which was the upper region.
The upper dark area was close to the surface and often came into contact with surface creatures. The water was abundant here, and food was relatively plentiful, making it a much more favorable condition compared to the deeper layers. It was also where numerous underground cities were located.
Black Rock City was a prime example of this.
