The shapes of size distributions of molecular clusters in highly supersaturated vapors, assuming they remain unrelaxed, i.e., in "constrained equilibrium," were derived for three representative systems. The required thermochemical parameters were estimated via the kinetic molecular model that accurately reproduced the measured condensation fluxes over a wide range of temperatures and supersaturation levels. The computed size distributions are neither log-normal nor follow an exponential-type decline with increasing cluster size.