The refractive index of Na2MgSi4O10 and Na2Al2Si4O12 glasses in the pressure range to 6.0 GPa was measured using a polarization-interference microscope and apparatus with diamond anvils. The changes in relative density, which characterize the compressibility of glasses, were calculated in the studied range of pressures from the measured refractive indices within the framework of the theory of photoelasticity. For the studied glasses, the degree of depolimerization was found to be NBO/T = 0.28 and 0.25 for Na2MgSi4O10 and Na2Al2Si4O12 glasses, respectively. It is shown that the replacement of aluminum by magnesium in silicate glasses does not lead to a considerable decrease in compressibility, which suggests the predominant role of magnesium ions as network formers. A minor increase in compressibility when passing from Na2MgSi4O10 to Na2Al2Si4O12 glass conforms with the corresponding decrease in the degree of depolymerization.