The density and spatial distribution of oxide precipitates within a crystalline silicon wafer is of paramount importance for microelectronic device yield. In this letter, the authors show how the formation of previously unconsidered, very small vacancy aggregates can explain macroscopic spatial variations in the oxide precipitate density, which are observed following certain rapid thermal annealing conditions. The formation of these nanometer-sized voids is predicted on the basis of their recent model for vacancy aggregation that accounts for high temperature entropic effects.