  The direct correspondence between Co band ferromagnetism and structural
parameters is investigated in the pnictide oxides $R$CoPO for different
rare-earth ions ($R$ = La, Pr, Nd, Sm) by means of muon-spin spectroscopy and
{\it ab-initio} calculations, complementing our results published previously
[G. Prando {\it et al.}, {\it Phys. Rev. B} {\bf 87}, 064401 (2013)]. Both the
transition temperature to the ferromagnetic phase $T_{_{\textrm{C}}}$ and the
volume of the crystallographic unit cell $V$ are found to be conveniently tuned
by the $R$ ionic radius and/or external pressure. A linear correlation between
$T_{_{\textrm{C}}}$ and $V$ is reported and {\it ab-initio} calculations
unambiguously demonstrate a full equivalence of chemical and external
pressures. As such, $R$ ions are shown to be influencing the ferromagnetic
phase only via the induced structural shrinkage without involving any active
role from the electronic $f$ degrees of freedom, which are only giving a
sizeable magnetic contribution at much lower temperatures.
