When a dog digs a hole, where does the dirt go?
My Jack Russell Terrier digs holes about 6 inches deep, and there is not a corresponding 6 inch pile of dirt. 
It’s probably scattered all around the hole. If he had a shovel, you’d see a pile.
It’s hiding in his fur, ready to escape once he climbs on the couch 
The resulting height of the dirt (assume it’s evenly spread) is its volume divided by the area that it gets spread over.
If you double both the width and the length of an area, it gets four times bigger, or, the area of a shape changes with the change of size squared.
It only takes about 5x5 feet to get a cube of 6x6x6 inches down to a 1/16 inch thick layer.