Buildup history of UFDs and their final shape. Left: evolution of cluster size in proper units for the four models h277, h249, h315, and h323. Each line corresponds to one cluster and follows the colour code of Fig. 2, the red one being the main cluster, defined as the one with the largest number of particles. The dotted line at the bottom of the plot indicates the resolution of the simulation, i.e. the gravitational softening length. The horizontal thick line corresponds to the final size of the entire stellar system, i.e. considering all clusters together. Right: two projections of the final stellar distribution of the UFDs at z = 0. Particles are coloured according to their membership to clusters. The grey map in the background corresponds to the DM surface density.