Oxygenic photogranules (OPGs), spherical aggregates comprised of phototrophic and nonphototrophic microorganisms, treat wastewater without aeration, which currently incurs the highest energy demand in wastewater treatment. In wastewater-treatment reactors, photogranules grow in number as well as in size. Currently, it is unknown how the photogranules grow in size and how the growth impacts their properties and performance in wastewater treatment. Here, we present that the photogranules’ growth occurs with changes in phototrophic community and granular morphology. We observed that as the photogranules grow larger, filamentous cyanobacteria become enriched while other phototrophic microbes diminish significantly. The photogranules greater than 3 mm in diameter showed the development of a layered structure in which a concentric filamentous cyanobacterial layer encloses noncyanobacterial aggregates. We observed that the growth of photogranules significantly impacts their capability of producing oxygen, the key element in OPG wastewater treatment. Among seven size classes investigated in this study, photogranules in the 0.5–1 mm size group showed the highest specific oxygen production rate (SOPR), 21.9 ± 1.3 mg O2/g VSS-h, approximately 75% greater than the SOPR of mixed photogranular biomass. We discuss engineering the OPG process based on photogranules’ size, promoting the stability of the granular process and enhancing efficiency for self-aerating wastewater treatment.