  The radii of young (<100 Myr) star clusters correlate only weakly with their
masses. This shallow relation has been used to argue that impulsive tidal
perturbations, or `shocks', by passing giant molecular clouds (GMCs)
preferentially disrupt low-mass clusters. We show that this mass-radius
relation is in fact the result of the combined effect of two-body relaxation
and repeated tidal shocks. Clusters in a broad range of environments including
those like the solar neighbourhood evolve towards a typical radius of a few
parsecs, as observed, independent of the initial radius. This equilibrium
mass-radius relation is the result of a competition between expansion by
relaxation and shrinking due to shocks. Interactions with GMCs are more
disruptive for low-mass clusters, which helps to evolve the globular cluster
mass function (GCMF). However, the properties of the interstellar medium in
high-redshift galaxies required to establish a universal GCMF shape are more
extreme than previously derived, challenging the idea that all GCs formed with
the same power-law mass function.
