The specimen size has a significant influence on the flow behavior of Zr55Cu30Al10Ni5 bulk metallic glass throughout thermoplastic compression processes in the supercooled liquid region. Due to enhanced structural relaxation, the recovery enthalpy increases with specimen size decreasing. Based on combined differential scanning calorimetry experiments and finite element modeling simulations, the different free volume annihilations caused by the specific surface area discrepancy are considered as the main cause of size-dependent free volume variation during the pre-heating process. Thermodynamics and structural relaxation theory are introduced to provide a reasonable explanation of the size-dependent flow behavior at elevated temperatures. A novel prediction model involving size-induced free volume variations has been established by considering the influences of compression deformation variables. The quantitative theoretical prediction agrees well with experimental data, which indicates that the developed prediction model can accurately describe the size-dependent free volume annihilation and generation during the thermoplastic forming process. Furthermore, the reasonable theoretical analysis and size-dependent free volume prediction model provide a profound understanding of the size-dependent flow behavior and its relation to internal structural evolution behaviors of metallic glasses. (C) 2016 Elsevier Ltd. All rights reserved.