We examine thin film on a dielectric substrate (vacuum/Al/SiO$_{2}$) in the stabilized jelly model. We investigate the surface and size effects on the effective potential and the electron work function for the weak quantization regime. We find that a dielectric environment generally leads to the decrease of the work function. We introduce the position of a conduction band for the dielectric as the input parameter in the self-consistency procedure. The effect of dielectric confinement for the energy characteristics of the asymmetric metal–dielectric sandwiches is reduced to only by the surface-area weighted average value of the dielectric constants. This conclusion follows from the application of the Gauss theorem for a conducting sphere with an inhomogeneous dielectric coating. The flow of electrons from the dielectric face to the vacuum one due to the contact-potential difference manifests itself in the appearance of a potential barrier above the vacuum level or positive values of the effective potential. The barrier height depends on the used local or non-local approximation of the exchange–correlation energy. The nontrivial origin and behaviour of the calculated effective potential on the vacuum side of the film, as well as the reasons for it, are discussed. In the focus of our representations, we analyse the recent results of measurements of the contact-potential difference depending on the number of Si atoms deposited on the free face of ytterbium nanofilms on the Si(111) substrates. Comparison and discrepancies between our self-consistent calculations for simple metals and these experiments are discussed.