The piezoelectric properties of [0001]-oriented ZnO nanowires are investigated via density functional theory (DFT). The axial effective piezoelectric coefficient of ZnO nanowires is significantly greater than the bulk value, and the coefficient increases as the nanowire size decreases. It is proved that the enhancement comes from both the reduction of volume per Zn-O pair and the enhancement of the Poisson's ratio. Further study shows that the macroscopic polarization behavior of ZnO nanowires is determined by the crystal structure parameters and the ratio of surface atoms, and an analytic expression is obtained. This work provides a deeper understanding of the size effects of the piezoelectricity of ZnO nanowires and sheds some light on the confusion reported on this subject.