GCMC simulations of adsorption of argon in slit-like pores show that adsorbents can swell or shrink with adsorbate loading. It is found that the shrinking or swelling depends on loading, pore size and temperature. The complex functional dependence is illustrated by adsorption in pores of various sizes at a number of temperatures. For pores where the adsorbate layers are not commensurate with pore width; for example when the pore width is 8A, shrinkage is observed under sub-critical conditions; while shrinkage followed by swelling occurs under supercritical conditions. The swelling occurs due to extremely high external pressure. However for commensurate pores that can accommodate an integer number of layers under normal conditions; for example when the pore width is 10A, we observe only moderate swelling at high pressures. For both sub- and supercritical conditions the dependence on loading is quite complex. When these results are compared with the equivalent rigid pores, we find that the difference in adsorption capacity, molar enthalpy of the adsorbed phase and solvation pressure is significant at high pressures under supercritical conditions.