Multicomponent diffusivities in zeolites are predicted using the framework of the theory of irreversible thermodynamics, for which the information on equilibrium mixture adsorption isotherms is a critical requirement. Two thermodynamically consistent mixture isotherms, a multi-site Langmuir model and a modified Dubinin-type expansion facilitate quantification of the thermodynamic effect. In both these isotherms, saturation capacities of the species are allowed to differ. The ratio of saturation capacities in a binary mixture represents, to a first approximation, the inverse of the ratio of molecular sizes. In this work, thermodynamic interactions based on the two isotherm models are isolated from kinetic interactions due to surface mobilities which may also depend on concentration, and applied to study the concentration dependence of the effective Fickian diffusivities of components in a binary mixture. For the special case of zero cross terms in the phenomenological diffusion coefficient matrix and constant surface mobilities, size difference and the choice of isotherm are both shown to have significant effects on binary diffusion in mixtures of unequal sized molecules. For the multi-site Langmuir isotherms, co-diffusion enhances while counter-diffusion diminishes diffusivity, as expected. The thermodynamic effect of the Dubinin-type isotherm, however, shows a different behavior for the effect of one species on the other. The size effect is amplified in the case of counter-diffusion, and leads to possible enhancement in diffusivity even in counter-diffusion.