This may be a wrong mental model, but I view it as overall cost to maintain a structure. For example, the number of nodes (volume / support) to build a pyramid

This may be a wrong mental model, but I view it as overall cost to maintain a structure. For example, the number of nodes (volume / support) to build a pyramid of height H scales non-linearly with H. Height is not the same as output (and H / V is not the same as efficiency), but this model has some similarity to the real world engineering output. Just my 2c.