  It has recently been shown that galaxy formation models within the LambdaCDM
cosmology predict that, compared to the observed population, small galaxies
(with stellar masses < 10^{11} M_sun) form too early, are too passive since z ~
3 and host too old stellar populations at z=0. We then expect an overproduction
of small galaxies at z > 4 that should be visible as an excess of faint
Lyman-break galaxies. To check whether this excess is present, we use the
MORGANA galaxy formation model and GRASIL spectro-photometric + radiative
transfer code to generate mock catalogues of deep fields observed with HST-ACS.
We add observational noise and the effect of Lyman-alpha emission, and perform
color-color selections to identify Lyman-break galaxies. The resulting mock
candidates have plausible properties that closely resemble those of observed
galaxies. We are able to reproduce the evolution of the bright tail of the
luminosity function of Lyman-break galaxies (with a possible underestimate of
the number of the brightest i-dropouts), but uncertainties and degeneracies in
dust absorption parameters do not allow to give strong constraints to the
model. Besides, our model shows a clear excess with respect to observations of
faint Lyman-break galaxies, especially of z_{850} ~ 27 V-dropouts at z ~ 5. We
quantify the properties of these "excess" galaxies and discuss the
implications: these galaxies are hosted in dark matter halos with circular
velocities in excess of 100 km s^{-1}, and their suppression may require a deep
re-thinking of stellar feedback processes taking place in galaxy formation.
