You can't understand it because you have it backward.
If land is cheap you have urban areas.
If land is expensive you have dense cities (since a small apartment is all you can afford).
Once you realize that cause and effect are the reverse of what you think, it will make more sense.
When land prices go up, lots get rapidly subdivided with more houses, and then apartments (as prices go up higher).
When land prices go down, buildings get torn down, and the (now vacant) adjacent yards get added to existing homes.