The effect of sample size on the compressive plasticity of (Zr₅₅Cu₃₀Al₁₀Ni₅)₉₉Nb₁ bulk metallic glass (BMG) has been studied on both as-cast and as-annealed samples. The results have revealed a size dependency of the mechanical behavior. The size effect has been considered according to two aspects: free volume and atomic mobility in shear bands. It is proposed that more shear softening in the largest sample as well as the more excess free volume in the sample experiencing higher cooling rate are responsible for the strong size effect. It is proposed that the amount of heat generated in shear bands is the most effective factor in deformation of BMGs