In the case of a scale-invariant type of system (where a system of volume  has exactly the same set of microstates as  systems of volume ), then when the system expands new particles and energy will flow in from the reservoir to fill the new volume with a homogeneous extension of the original system.
The pressure, then, must be constant with respect to changes in volume:
and all extensive quantities (particle number, energy, entropy, potentials, ...) must grow linearly with volume, e.g. 
In this case we simply have , as well as the familiar relationship  for the Gibbs free energy.
The value of  can be understood as the work that can be extracted from the system by shrinking it down to nothing (putting all the particles and energy back into the reservoir). The fact that   is negative implies that the extraction of particles from the system to the reservoir requires energy input.
Such homogeneous scaling does not exist in many systems. For example, when analyzing the ensemble of electrons in a single molecule or even a piece of metal floating in space, doubling the volume of the space does double the number of electrons in the material.
The problem here is that, although electrons and energy are exchanged with a reservoir, the material host is not allowed to change.
Generally in small systems, or systems with long range interactions (those outside the thermodynamic limit), .