Based on the accurate definition of light intensity of nonparaxial scalar beams at the transverse plane, a comparative study of the beam widths, far-field divergence angles and M2 factors defined by the power in the bucket and second-order moment is made. Taking the nonparaxial diffraction of a small circular aperture as an example, the detailed numerical calculation is performed. The results show that the beam width, far-field divergence angle and M2 factor defined by the power in the bucket are less than that defined by the second-order moment. Both M2 factors tend to zero as beam waist vanishes, the propagation of beam width defined by the power in the bucket is different from the hyperbolic law.