Spin-coated polystyrene films thinner than the unperturbed size of the molecules on glass substrates were studied by X-ray reflectometry. Detailed knowledge about the dewetting process of these unstable films allowed to probe polymer dynamics in this confined geometry. The film thickness increased upon annealing reaching a stable value before dewetting started. The thickness change translates into a decrease in polymer density. This decrease is larger the thinner the films are. Consequently, the polymer density depends on film thickness and reaches its bulk value only for films thicker than about the size of the unperturbed molecule. A quite distinct dependence of the onset of dewetting on film thickness was observed. For the thinnest films dewetting was completed at temperatures well below the glass transition temperature. As a consequence of the lower polymer density, the mobility of the molecules was increased. A tentative explanation of these effects based on packing considerations leading to inaccessible sites is given.