Experimental evidence and a simple model for a special segregation effect in hydrophobically initiated emulsion polymerization are presented. This effect is controlled by the size ratio between the droplets and the perfectly mixed volume of the growing chains. It leads within a certain range of droplet size to a drastic retardation of the polymerization reaction. For reaction loci outside this critical range, the monomer conversion is, at given polymerization time, drastically enhanced. The higher conversion for droplets that are smaller than the critical value can be explained by the effective segregation of growing radicals as typical for emulsion polymerization. Increasing conversion in bigger sized droplets happens as soon as the droplet volume exceeds the corresponding perfectly mixed volume. The limiting case of the latter situation is been experimentally observed for the corresponding bulk polymerization. Mathematical simulations with a simple model taking into account imperfectly mixed conditions inside the droplets describe the essential experimental data astonishingly well.