Lead halide perovskite (APbX(3)) has recently emerged as a promising active layer in light-emitting diodes (LEDs) as well as an absorber for photovoltaic devices. For better LED properties, it is important to understand the fundamental mechanism of the optoelectronic behaviors, e.g., how the nanostructure of the APbX(3) thin film correlates with its emitting properties. We investigated the effect of APbBr(3) (A = CH3NH3, Cs) crystallite size on the photophysical properties regarding its crystallographic changes and spin-orbit coupling. Photoluminescence lifetime measurements, Xray and electron diffraction analyses, and density functional theory calculations were performed. We demonstrate that the emitting properties of mesoscale APbBr(3) crystallites are improved due to the formation of a pure cubic phase that leads to the spin- and momentum-allowed carrier recombination. Our findings provide fundamental insights into the emitting behavior of APbBr(3), which suggests a control of its optoelectronic properties by means of modulating the crystal morphology and resultant electronic band structures.