And just WHO is going to live there? The population of Saudi Arabia isn’t enough to fill a city that gigantic.
Saudi population is over 20 million (about 5 million are guest workers, who do most of the work in the country); the area looks excessive, but A) even with their declining birthrate, a place of that kind is bound to attract Arabs from neighboring areas, such as "Palestinians" from Jordan, Egypt, Gaza, Israel, and the other countries in the Middle East where they live; and B) a good bit of the area will be non-residential, iow, commercial, high tech manufacturing, and infrastructure (water desalination, photovoltaic arrays, power storage).