NVT Monte Carlo simulations of highly asymmetric square well mixture systems with size ratio of 4-8, have been performed. The coordination numbers, internal energies, compressibility factors and the radial distribution functions (RDFs) are reported. Simulations suggest that at least 2.0 x 10(7) configurations must be generated for obtaining smooth RDFs for the systems. Simulated and calculated coordination numbers and compressibility factors from several coordination number models (CNMs) and equations-of-state (EOSs) are compared. The results indicate that our EOS is of comparable accuracy with the second order perturbation theory (PT2) EOS, and gives a better fit to the simulated data than the other two CNM EOSs. More importantly, we compare our EOS with the others for the systems at size ratios of 4.0, 6.0, 8.0, and composition x(1) = 0.9 (species I is referred to small molecule in the system), which can be considered as reference fluids for colloidal systems. lt is found that our CNM EOS is, in general, of better accuracy. The other CNM EOSs almost always overestimate the compressibility factors from low to high densities, while the PT2 EOS underestimates them with pronounced discrepancies in the reduced density range, rho* = 0.2-0.7, even though that it gives good agreement at very high density rho* = 0.8.