Positronium lifetimes in bisphenol A-polycarbonate (PC) and atactic polystyrene (PS) were measured as a function of temperature and hydrostatic pressure, and the results were compared with the macroscopic volume change of the polymer samples. A simple relation via the thermal expansion coefficient and the compressibility of the bulk material is applied and leads to direct information on the fractional free volume of the polymer in the investigated state and furthermore to the mean volume size of intermolecular holes. By plotting macroscopic volume changes vs changes of the hole volume as probed by the positronium, a linear relationship is obtained independent of whether the measurements were made above or below the glass transition temperature. The slope of the corresponding straight lines is steeper for samples exposed to hydrostatic pressure which is interpreted as a shrinkage of holes induced by pressure.