@Viraam_Rao tell me if this seems plausible. Frenkel defect is a type of interstitial and vacancy defect both. The cation shifts to an interstitial site, and creates a vacancy in its original site, in frenkel defect. However, there is no other foreign particle present in any site.
But, in non ionic solids, electrical neutrality does not matter. In the vacancy created due to shifting to interstitial site, what if a foreign particle settles in. Kind of like doping. That would increase the mass, keeping the volume same, wouldn’t it? But in frenkel, the vacant site remains vacant. Hence no increase in mass or volume.
This is what came to mind.