The optimization of nonlinear optical processes in plasmonic structures is important for the design of efficient nonlinear nanosources of light. Considering the simple case of spherical nanoparticles, we clearly identify the most efficient channel for second-harmonic generation, thanks to physical insights provided by the generalized Mie theory. This channel corresponds to the excitation of electric dipolar modes at the fundamental wavelength and a quadrupolar second-harmonic emission. Interestingly, it is demonstrated that the second-harmonic generation intensity is directly related to the square of the absorbed power, which reproduces both the electric field enhancement and the specific size dependence of second-harmonic generation in the small-particle limit. Additionally, the absorbed power can be optimized by controlling the nanoparticle size. These results demonstrate that the optimization of the fundamental electric field is not sufficient for reaching the highest nonlinear conversion in plasmonic systems. The approach reported in this article proposes a new paradigm for the design of nonlinear plasmonic nanostructures, establishing new rules for the conception of efficient nonlinear plasmonic metamolecules on the basis of their linear response.