Density changes of GeO2 and SiO2 glasses subjected to irradiation by tightly focused femtosecond pulses are observed by Raman scattering. It is shown that densification caused by the void formation in GeO2 glass is very similar to the changes under hydrostatic pressure. In contrast, the experimental observations in SiO2 glass could be explained by pressure effect or by the fictive temperature anomaly, i. e., a resultant smaller specific volume of the glass (a denser phase) at a high thermal quenching rate. Density changes of GeO2 and SiO2 glasses are opposite upon close-to-equilibrium heating; this gives new insights into the mechanisms of densification under highly non-equilibrium conditions: fs-laser induced micro-explosions, heating and void formation. The pressure and temperature effects of glass modification by ultra-short laser pulses are discussed considering applications in optical memory, waveguiding, and formation of micro-optical elements.