  Two basic routes for planetesimal formation have been proposed over the last
few decades. One is a classical "slow-growth" scenario. Another one is particle
concentration models, in which small pebbles are concentrated locally and then
collapse gravitationally to form planetesimals. Both types of models make
certain predictions for the size spectrum and internal structure of newly-born
planetesimals. We use these predictions as input to simulate collisional
evolution of debris discs left after the gas dispersal. The debris disc
emission as a function of a system's age computed in these simulations is
compared with several Spitzer and Herschel debris disc surveys around A-type
stars. We confirm that the observed brightness evolution for the majority of
discs can be reproduced by classical models. Further, we find that it is
equally consistent with the size distribution of planetesimals predicted by
particle concentration models - provided the objects are loosely bound "pebble
piles" as these models also predict. Regardless of the assumed planetesimal
formation mechanism, explaining the brightest debris discs in the samples
uncovers a "disc mass problem." To reproduce such discs by collisional
simulations, a total mass of planetesimals of up to ~1000 Earth masses is
required, which exceeds the total mass of solids available in the
protoplanetary progenitors of debris discs. This may indicate that stirring was
delayed in some of the bright discs, that giant impacts occurred recently in
some of them, that some systems may be younger than previously thought, or that
non-collisional processes contribute significantly to the dust production.
