Nanostructurization effects, in glassy arsenoselenides g-As_xSe_100-x (5 ≤ x ≤ 65) subjected to wet nanomilling in polyvinylpyrrolidone (PVP) water solution, are studied employing the Positronics, an advanced characterization tool tracing free-volumetric entities in solids based on the positron annihilation lifetime (PAL) spectroscopy. Straightforward interpretation of the results is developed due to the XRD analysis recognizing microstructural peculiarities of medium-range ordering in PVP-nanosized g-As-Se in terms of modified microcrystalline approach. Nanomilling is shown to be ineffective in under-stoichiometric g-As_xSe_100-x (x < 40) possessing network structures, while causes reamorphization in over-stoichiometric glass (x > 40) due to molecular-to-network transition. Compositional changes in the PAL spectra reconstructed under unconstrained three-component fitting are revealed in under-stoichiometric g-As_xSe_100-x/PVP nanocomposites (x < 40), showing depressing trend in positionally-invariant PAL spectrum peaks supplemented by widely expanded tails composed of accumulated long-lived annihilation events. These changes are merely suppressed in over-stoichiometric nanocomposites (x > 40). Preliminary analysis within two-state simple trapping model, testifies that positron trapping occurs in intrinsic voids of g-As-Se enlarged to the character volumes of multiatomic vacancies, while bound positron–electron (positronium, Ps) states decay in PVP medium. Formalism of × 3- × 2-CDA (coupling decomposition algorithm) describing conversion of Ps-related states into positron traps is applied to identify volumetric changes in nanocomposites in respect to stoichiometric g-As₂Se₃/PVP. The governing process in under-stoichiometric nanocomposites (x < 40) is recognized as unified positron-to-Ps trapping conversion, disappearing positron traps being PVP-nanosized vacancies character to g-As-Se. However, there is no alone trapping-modification process governing such effects in over-stoichiometric nanocomposites (x > 40), possessing molecular-network structure with principally different trapping sites.