Spatially resolved photoluminescence (PL) investigations were carried out on ZnO single nanowires, tetrapods, and nanocrystals. The fractional intensity for bound exciton (BX) transitions was shown to be correlated with the size in all these ZnO nanostructures. This size dependency is attributed to the inhomogeneous density distribution of the defects as binding sites for BX in the ZnO nanostructures, in good agreement with a simple model calculation.