We report the results of classical, grand canonical Monte Carlo simulations of the physisorption of H-2 in finite-diameter 'ropes' of parallel single-walled carbon nanotubes. The strong dependence of the gravimetric adsorption on the diameter of the rope is correlated with computed values of the specific surface area. The grooves on the external surfaces of the ropes are shown to provide a high binding-energy adsorption site, comparable in strength to those in the endohedral pores. Our results suggest that delamination of nanotube ropes should increase the gravimetric storage capacity.