This paper provides an overview of the influence of the phonon subsystem onto the mechanical and thermodynamic properties of low-dimensional materials (particularly, ultrathin films), which has been the subject of our team's research for many years. Explorations of that kind have become very important for the materials research science and engineering over the last few years, due to the great commercialization potential of the novel two-dimensional structures, such as graphene. It is shown how the quantum size effect influences the change of physical properties of the above-mentioned structures and the consequences of this impact have been discussed in detail. Theoretical analysis has been conducted by means of the method of two-time dependent Green's functions by which the spectra of allowed phonon energies, as well as thermodynamic characteristics of these structures, has been determined. The diffusion processes have been treated by means of Kubo formula, adapted to the phonon subsystems in nanostructures.