what is the "small particle" explination for why matter conserves mass but not volume when mixed?

Answer 1

intermolecule or inter atomic distances can vary depending on the type of attraction in between them, thus volume is not conserved. hoewever, the mass would not change since that amount of substance you put in didn't change.
However, ideally there can be a chaqnge in mass if there is a absorption or release of energy, but is negligible in normal cases but detectable in nuclear reactions.

Answer 2

It isn't really the particle itself but how that particle behaves in space, how it affects the space around it, electronically and "otherwise" (Gravitationally but science isn't quite there yet)