Adsorbed solutions of molecules of unequal size (CCl2F2 and CO2 on activated carbon) show negative deviations from ideal mixing. The size effect was studied by molecular simulation of a Lennard-Jones fluid mixture adsorbed in zeolite 13X. The molecules forming the binary mixture are non-polar and have a 2.4 ratio of saturation capacities (the same as CC2F2 and CO2) In the molecular simulation, adsorption occurs on the energetically homogeneous surface of a spherical shell. Binary adsorption is centered on separate shells with the larger molecule in the inner shell. At high coverage, the simulations yield moderate negative deviations from ideal mixing similar to experimental data. Both molecules can enter the cavity, but adsorbed-phase nonidealities are caused by the exclusion of the larger molecule from part of the micropore space accessible to the smaller molecule. The partial exclusion effect is calculated from the cavity radius and molecular diameters of the adsorbate molecules.