The kinetics of size segregation in quasi-two-dimensional binary mixtures of nearly hard-sphere colloids were studied with video microscopy. During the transient fluid-fluid phase separation that occurs as an intermediate step in the formation of isolated large-sphere crystallites, the structure factor of the (larger) minority component exhibits a spinodal-like evolution, while the cluster-size distribution exhibits scaling reminiscent of colloidal aggregation. The scaled distributions suggest a crossover from power-law (x < 1) to stretched-exponential (x > 1) behavior, where x = k/s is the ratio of the cluster-size index to the average cluster size. A phenomenological explanation based on the reversible Smoluchowski equation is proposed.