Depletion-force kinetics in confined nearly hard-sphere mixtures are interpreted within the context of reversible two-dimensional aggregation. The cluster-size distribution function of the minority component exhibits an exponential decay in the cluster size, with a time-dependent decay constant that relaxes toward the steady-state value suggested by simulations of two-dimensional evaporation-condensation phenomena with Brownian (random-walk) trajectories. This similarity is attributed to a greatly reduced mobility of the aggregates with respect to unbound large spheres.