Based on the statistical data of first pop-in events during nanoindentation, the shear transformation zone (STZ) volumes of metallic glassy films with various compositions were estimated through the cooperative shearing model (CSM). The experimental results indicate that the STZ volume decreases with Poisson’ ratio of the sample, which contradicts with previous observations using the rate-jump method. Moreover, the critical size for deformation mode transition (i.e. the localized shearing to homogeneous flow) at nanoscale could intrinsically be correlated with STZ volume.