X-ray diffraction and ultrasonic velocity measurements of three silicate glasses (in jadeite, albite, and diopside compositions) show a sharp contrast in pressure-induced changes in structure and elasticity. With increasing pressure to around 6 GPa, polymerized glasses (jadeite and albite) display large shift in the first sharp diffraction peak (FSDP) in the structure factor, S(Q)S(Q), to higher-Q values, indicating rapid shrinkage in the intermediate-range ordered (IRO) structure. Above 6 GPa, the shift of FSDP decelerates, suggesting that shrinkage in the IRO structure has been largely completed and the structure evolution is now dominated by the diminution of the interstitial volume in a more densely packed arrangement. Associated with this structural change, sound velocities increase with pressure above 6 GPa. In contrast, the depolymerized diopside glass exhibits smaller changes in the pressure dependence for both sound velocities and FSDP positions. Compared to the polymerized glasses, the velocities are faster and the positions of FSDP appear at higher-Q under the same experimental conditions. The results suggest that the depolymerized diopside glass has an initially denser IRO structure compared to that of the polymerized glasses, and there are no sufficient interstitial voids to shrink. The different behaviors between polymerized and depolymerized glasses are apparently related to the initial linkage of tetrahedra and the pressure-induced structural reactions. These results suggest that under compression up to 10 GPa, the degree of polymerization is a major factor affecting the IRO network structure and the sound velocity of silicate glasses.