Droplet grouping/clustering can occur under appropriate operating conditions when droplets are travelling in an oscillating host gas flow. Previous work revealed the impact this phenomenon has on the evolutionary behavior of laminar co-flowing spray flames. In the current work we consider a realistic polydisperse fuel spray. We suggest a model that captures differential droplet size clustering and, using it, examine in some detail how this droplet size effect influences the internal spray evolution thereby altering the flame dynamics. The results indicate that the differential droplet size effect is not negligible.