A simple model is used to calculate the tracer diffusion in a Lennard-Jones tracer-solvent system. The influence of the size ratio between tracer and solvent, and of their mass ratio on the tracer diffusivity is investigated. The adopted model, proposed by Tankeshwar et al., is based on the idea, put forward by Zwanzing, of separating the configurational space of fluid system into a vibrational part and a structural part. The results offer an interpretation of the molecular dynamics data showing a positive deviation from Stokes-Einstein behaviour for small size of the tracer. The deviation is explicitly related to the length parameter of tracer-solvent interaction. The calculated values of the tracer diffusion coefficients for a large range of its size and mass values are quite satisfactory as judged by comparison with simulation results. (author). 12 refs, 4 figs, 1 tab.