Using the general bulk thermodynamics and a molecular equation-of-state model, we provide a heuristic argument on driving forces for phase segregation in polymer mixtures. Particular attention is given to the immiscibility loop emerged under pressure for a binary mixture of polymers of relatively low sizes, which differ in their compressibility. As the disparity in the equation-of-state properties governs the entire phase diagram for a given system, such compressibility effect alters its character as being entropically or enthalpically contributing to phase segregation according to the lower or upper critical solution (order-disorder) transition.