This study investigates the size-dependent mechanical properties of electrospun polycaprolactone (PCL) nanofibers by analyzing the relationship between fiber diameter and Young's modulus. Experimental data reveal a clear inverse trend: as fiber diameter decreases, stiffness increases significantly, indicating strong surface and confinement effects at the nanoscale. Two theoretical models were employed to interpret the observed behavior: a simplified core–shell model (Model 1) and an extended model (Model 2) incorporating surface tension and curvature elasticity. Both models accurately fit the experimental data across a diameter range of 450–850 nm, with Model 2 providing slightly better agreement at intermediate diameters (∼600–750 nm), where surface mechanics become more prominent. The enhanced stiffness in thinner fibers is attributed to increased surface-to-volume ratio and tighter molecular packing, while larger fibers exhibit bulk-dominated mechanical responses. These findings highlight the importance of nanoscale geometry and surface effects in determining mechanical properties and suggest that fiber stiffness can be systematically tuned via diameter control during electrospinning.