An equation of state following from the generalized Lennard-Jones potential has been successfully applied to describe isothermal volumetric data for several glass-forming liquids on the assumption of the thermodynamic scaling validity. The scaling exponents gamma evaluated from the fitting procedure for each tested material have turned out at least 2 times larger than those earlier found from relaxation data. The discrepancy indicates that the appealing idea of thermodynamic scaling, which has been recently intensively explored, requires more efforts to find a satisfying theoretical grounds for it.