Abstract Nanoscale polymeric thin films are widely used in diverse modern applications, where a satisfactory mechanical performance is a requirement to their full functionality. The mechanical response of polymer films is strongly affected by the size effects under nanoconfinement; however, the mechanism of such response in terms of molecular configurations and chain conformations has yet to be determined. In this work, we reveal the conformational origin of the stiffening behavior of crosslinked polymeric nanofilms via coarse-grained molecular dynamics and tailored experiments. We find that the biaxial modulus changes follow the alteration of polymer conformations, decoupled from size and thickness. We propose a theory to quantitatively link the elastic properties of the polymers to the distribution of their chain end-to-end distances, predicting a stiffening effect on uncoiled chains. Finally, we use such insight to obtain several PDMS nano-films of the same thickness but with a variability of two orders of magnitudes in their moduli.