I have read that when representing the possible nuclides in the $(Z,N)$ plot, the stable nuclei located on the line $N=Z$ for $A<40$, whereas they are located on the line $N=1.56Z$ for $A>40$.

I understand that the tendency to present a greater number of neutrons than protons for $A>40$ is due to the fact that they provide strong nuclear force with which they compensate the Coulomb repulsion of the protons. However, I have read that the tendency to present $Z=N$ for $A<40$ is due to the Pauli exclusion principle, which is not clear to me. How does this principle affect the ratio of neutrons to protons in the nuclides?

I have read that when representing the possible nuclides in the $(Z,N)$ plot, the stable nuclei located on the line $N=Z$ for $A<40$, and that this is due to the Pauli exclusion principle. 

I have read about it in this question, but this is still not entirely clear to me: why does the Pauli exclusion principle imply that nuclides with an unequal proportion of protons and neutrons are unstable?