Amorphous oxide thin films are crucial materials for micro- and nanoelectronics device fabrication and have frequently been employed as a component of device stacks (e.g. gate dielectrics), as well as hard masks for lithography and protective layers for ion etching. In this work, we report thin films of a-SiOx deposited on Si substrates using reactive magnetron sputtering techniques. By adjusting the oxygen content and silicon magnetron power during the deposition process we found that it is possible to obtain films with a density up to 7% lower and up to 25% higher compared to the bulk counterpart (rho = 2.20 g cm-3). Through first-principles density functional theory simulations, we investigated the formation of native point defects in amorphous silicon dioxide ( a-SiO2). Radial distribution function analysis of defective a-SiO2 revealed substantial changes in the structural properties due to defect formation. We thus provide an atomistic explanation and understanding for the significant variation in mass density and correlate it with the different point defect formations that occur under varying deposition conditions.