  We interpret the relation between the polarizing efficiency $P_{\max}/E(B-V)$
and the wavelength of the maximum polarization $\lambda_{\max}$ observed for 17
objects (including 243 stars) separated into two groups: "dark clouds" and
"open clusters". The objects are assigned to one of the groups according to the
distribution of the parameter $\lambda_{\max}$. We use the model of homogeneous
silicate and carbonaceous spheroidal particles with the imperfect alignment and
a time-evolving size distribution. The polarization is assumed to be mainly
produced by large silicate particles with the sizes $r_{V} \ga r_{V,\rm cut}$.
The models with the initial size distribution reproducing the average curve of
the interstellar extinction fail to explain the values of $\lambda_{\max} \ga
0.65\,\mkm$ observed for several dark clouds. We assume that the grain size
distribution is modified due to accretion and coagulation, according to the
model of Hirashita \& Voshchinnikov (2014). After including the evolutionary
effects, $\lambda_{\max}$ shifts to longer wavelengths on time-scales $\sim 20
(n_\mathrm{H}/10^3 \mathrm{cm}^{-3})^{-1}$ Myr where $n_\mathrm{H}$ is the
hydrogen density in molecular clouds where dust processing occurs. The ratio
$P_{\max}/E(B-V)$ goes down dramatically when the size of polarizing grains
grows. The variations of the degree and direction of particle orientation
influence this ratio only moderately. We have also found that the aspect ratio
of prolate grains does not affect significantly the polarizing efficiency. For
oblate particles, the shape effect is stronger but in most cases the
polarization curves produced are too narrow in comparison with the observed
ones.
