  The evolution of the dust grain size distribution has been studied in recent
years with great detail in cosmological hydrodynamical simulations taking into
account all the channels under which dust evolves in the interstellar medium.
We present a systematic analysis of the observed spectral energy distribution
of a large sample of galaxies in the local universe in order to derive not only
the total dust masses but also the relative mass fraction between small and
large dust grains (DS/DL). Simulations reproduce fairly well the observations
except for the high stellar mass regime where dust masses tend to be
overestimated. We find that ~45% of galaxies exhibit DS/DL consistent with the
expectations of simulations, while there is a sub-sample of massive galaxies
presenting high DS/DL (log(DS/DL)~-0.5), and deviating from the prediction in
simulations. For these galaxies, which also have high molecular gas mass
fractions and metallicities, coagulation is not an important mechanism
affecting the dust evolution. Including diffusion, transporting large grains
from dense regions to a more diffuse medium where they can be easily shattered,
would explain the observed high DS/DL values in these galaxies. With this study
we reinforce the use of the small-to-large grain mass ratio to study the
relative importance of the different mechanisms in the dust life cycle.
Multi-phase hydrodynamical simulations with detailed feedback prescriptions and
more realistic subgrid models for the dense phase could help to reproduce the
evolution of the dust grain size distribution traced by observations.
