A statistical thermodynamic theory is presented with a new approach to account for the mixing of polymers, small molecules, and holes, which are different in size. The criterion for glass transition is taken to be the vanishing of entropy pertaining to the non-local movements of polymer segments rather than the total mixing entropy of the system. The theory adequately models the pressure-dependence of the glass transition temperature (T-g) of polymers as well as the dependence of T-g on plasticizer concentration. A molecular interpretation has also been given to the T-g of different pure polymers and to the efficiency of T-g reduction due to plasticizers in terms of several model parameters including intermolecular interaction, segmental volume, co-ordination number for the unplasticized polymer, polymer-plasticizer interaction, and plasticizer close-packed volume for the plasticized polymer.