The size-dependent plastic deformation of a Zr-based metallic glass under biaxial loading was investigated by using small punch (SP) test. Unlike under uniaxial tension or compression, under biaxial loading, both the critical shear offset and the density of shear bands decrease with the reduction of the sample size. However, it was found that the normalized critical shear offset keeps constant, which can well explain the worsened plastic deformation behaviors of the thin sample under biaxial loading. This finding indicates that metallic glass possesses good ductility in nature, yet it is influenced significantly by the loading mode and sample size.