  The stabilized jellium model (SJM) provides us a method to calculate the
volume changes of different simple metals as a function of the spin
polarization, $\zeta$, of the delocalized valence electrons. Our calculations
show that for bulk metals, the equilibrium Wigner-Seitz (WS) radius, $\bar
r_s(\zeta)$, is always a n increasing function of the polarization i.e., the
volume of a bulk metal always increases as $\zeta$ increases, and the rate of
increasing is higher for higher electron density metals. Using the SJM along
with the local spin density approximation, we have also calculated the
equilibrium WS radius, $\bar r_s(N,\zeta)$, of spherical jellium clusters, at
which the pressure on the cluster with given numbers of total electrons, $N$,
and their spin configuration $\zeta$ vanishes. Our calculations f or Cs, Na,
and Al clusters show that $\bar r_s(N,\zeta)$ as a function of $\zeta$ behaves
differently depending on whether $N$ corresponds to a closed-shell or an
open-shell cluster. For a closed-shell cluster, it is an increasing function of
$\zeta$ over the whole range $0\le\zeta\le 1$, whereas in open-shell clusters
it has a decreasing behavior over the range $0\le\zeta\le\zeta_0$, where
$\zeta_0$ is a polarization that the cluster has a configuration consistent
with Hund's first rule. The resu lts show that for all neutral clusters with
ground state spin configuration, $\zeta_0$, the inequality $\bar
r_s(N,\zeta_0)\le\bar r_s(0)$ always holds (self-compression) but, at some
polarization $\zeta_1>\zeta_0$, the inequality changes the direction
(self-expansion). However, the inequality $\bar r_s(N,\zeta)\le\bar r_s(\zeta)$
always holds and the equality is achieved in the limit $N\to\infty$.
