  Building galaxy merger trees from a state-of-the-art cosmological
hydrodynamics simulation, Horizon-AGN, we perform a statistical study of how
mergers and smooth accretion drive galaxy morphologic properties above $z > 1$.
More specifically, we investigate how stellar densities, effective radii and
shape parameters derived from the inertia tensor depend on mergers of different
mass ratios. We find strong evidence that smooth accretion tends to flatten
small galaxies over cosmic time, leading to the formation of disks. On the
other hand, mergers, and not only the major ones, exhibit a propensity to puff
up and destroy stellar disks, confirming the origin of elliptical galaxies. We
also find that elliptical galaxies are more susceptible to grow in size through
mergers than disc galaxies with a size-mass evolution $r \prop M^{1.2}$ instead
of $r \prop M^{-0.5} - M^{0.5}$ depending on the merger mass ratio. The gas
content drive the size-mass evolution due to merger with a faster size growth
for gas-poor galaxies $r \prop M^2$ than for gas-rich galaxies $r \prop M$.
