  The cross section of material in debris discs is thought to be dominated by
the smallest grains that can still stay in bound orbits despite the repelling
action of stellar radiation pressure. Thus the minimum (and typical) grain size
$s_\text{min}$ is expected to be close to the radiation pressure blowout size
$s_\text{blow}$. Yet a recent analysis of a sample of Herschel-resolved debris
discs showed the ratio $s_\text{min}/s_\text{blow}$ to systematically decrease
with the stellar luminosity from about ten for solar-type stars to nearly unity
in the discs around the most luminous A-type stars. Here we explore this trend
in more detail, checking how significant it is and seeking to find possible
explanations. We show that the trend is robust to variation of the composition
and porosity of dust particles. For any assumed grain properties and stellar
parameters, we suggest a recipe of how to estimate the "true" radius of a
spatially unresolved debris disc, based solely on its spectral energy
distribution. The results of our collisional simulations are qualitatively
consistent with the trend, although additional effects may also be at work. In
particular, the lack of grains with small $s_\text{min}/s_\text{blow}$ for
lower luminosity stars might be caused by the grain surface energy constraint
that should limit the size of the smallest collisional fragments. Also, a
better agreement between the data and the collisional simulations is achieved
when assuming debris discs of more luminous stars to have higher dynamical
excitation than those of less luminous primaries. This would imply that
protoplanetary discs of more massive young stars are more efficient in forming
big planetesimals or planets that act as stirrers in the debris discs at the
subsequent evolutionary stage.
