Run of the size of the young associations considered in this paper with age (red dots). X is the direction of the galactic centre (upper panel); Y is the direction perpendicular to X on the galactic plane (middle panel); Z is the vertical direction (lower panel). For comparison, expectations for a simple model of expansion of an association in the galactic potential is also shown; results for 200 random extractions for the initial sizes along X, Y, and Z axes with uniform distribution between 2 and 15 pc are shown. In all cases we assumed an initial isotropic velocity dispersion of 0.5 km s⁻¹.