Thin films of adsorbates on solid surfaces often exhibit irreversible clustering and island growth phenomena where the mean island size grows larger with a temporal power law dependence, accompanied by a scaling island size distribution function. This coarsening process is typically described within a thermodynamic framework using the Ostwald ripening formalism. However, there are strong indications that the Ostwald formulation is incomplete since it omits critical atomic level phenomena such as island mobility, spatial correlation between kinetic processes, and surface roughening of the islands. We have simulated thin film coarsening on an FCC(100) surface using a large Monte Carlo lattice gas model. Scaling exponents and island distribution functions were extracted from the simulations. From the Monte Carlo, we have computed rate constants for island evaporation-recondensation and island coalescence. Using a high-dimensional set of rate equations, a quasichemical mean field approach is formulated as a high dimensional set of second-order kinetics equations. The power law scaling behavior of the coarsening is reproduced by both the Monte Carlo simulations and the mean field theory. The relative importance of Ostwald theory versus island coalescence is evaluated.