I completely agree with you. Let's add to the fact that volume, being three-dimensional, is being represented on two dimensions (graphics on a computer screen),

I completely agree with you. Let's add to the fact that volume, being three-dimensional, is being represented on two dimensions (graphics on a computer screen), which might cause some loss of perspective, fundamental for comparison. Perhaps a better way to represent it would have been the volume of inhabitable land (as you suggest) vs the volume of available water but extrapolated to two dimensions?