It emphasizes neither!
What you've described is a mass grave.
Quite literally so. If you killed all living humans (8.3billion), the mass-grave you'd have to dig to put them all in one place isn't quite large indeed!
Plus, humans on earth are affected by gravity, so any arrangement of them cubic squares instead of square miles is highly unintuitive, unusual and unnatural to begin with.
This doesn't say anything about habitable area (measured in km^2, not in km^3) of the planet, or the number of people (that you've conveniently reduced by taking a square root of - twice! by packing them into a tightly packed cube)
For example, if you took 8 billion people and made them hold hands with each other tightly packed (0.5m per person) it would wrap the circumference of earth 100 times.
Now this actually says something about size of the earth!