  The presence of submicron grains has been inferred in several debris discs,
despite the fact that these particles should be blown out by stellar radiation
pressure on very short timescales. So far, no fully satisfying explanation has
been found for this apparent paradox. We investigate the possibility that the
observed abundances of submicron grains could be "naturally" produced in bright
debris discs, where the high collisional activity produces them at a rate high
enough to partially compensate for their rapid removal. We also investigate to
what extent this potential presence of small grains can affect our
understanding of some debris disc characteristics. We use a code following the
collisional evolution of a debris disc down to submicron grains far below the
limiting blow-out size $s_{blow}$. We explore different configurations: A and G
stars, cold and warm discs, "bright" and "very bright" systems. We find that,
in bright discs (fractional luminosity $>10^{-3}$) around A stars, there is
always a high-enough amount of submicron grains to leave detectable signatures,
both in scattered-light, where the disc's color becomes blue, and in the mid-IR
($10<\lambda<20\mu$m), where it boosts the disc's luminosity by at least a
factor of 2 and induces a pronounced silicate solid-state band around $10\mu$m.
We also show that, with this additional contribution of submicron grains, the
SED can mimic that of two debris belts separated by a factor of 2 in radial
distance. For G stars, the effect of $s<s_{blow}$ grains remains limited in the
spectra, in spite of the fact that they dominate the system's geometrical cross
section. We also find that, for all considered cases, the halo of small (bound
and unbound) grains that extends far beyond the main disc contributes to
$\sim50$% of the flux up to $\lambda\sim50\mu$m wavelengths.
