Diffusion of deuterated poly(methyl methacrylate) (dPMMA) is slowed down in a PMMA matrix filled with hydroxyl-capped spherical silica nanoparticles, from 13 to 50 nm in diameter and at loadings up to 40 vol %. At constant T - T-g = 75 K, the normalized diffusion coefficients (D/D-0) collapse onto a master curve, when plotted against the confinement parameter, ID/2R(g), where ID is interparticle distance and 2R(g) is probe size. This result suggests that the confinement parameter captures the effect of nanoparticle size, size polydispersity, and volume fraction on polymer dynamics for the PMMA composite. For ID < 2R(g), the master curve exhibits a strongly confined region where D/D-0 decreases by up to 80%, whereas for ID > 2R(g), the curve falls in a weakly confined region where D/D-0 decreases only moderately by up to 15%. Surprisingly, D/D-0 is reduced even when ID is 8 times larger than 2R(g). A comparison between the master curves for PMMA and polystyrene nanocomposites indicates that attractive interactions in the PMMA system do not significantly alter the center-of-mass diffusion of macromolecules in polymer nanocomposites.