We present the morphological evolution obtained during the annealing of Ge strips grown on Si ridges as a prototypical process for 3D device architectures and nanophotonic applications. In particular, the morphological transition occurring from Ge/Si nanostrips to nanoislands is illustrated. The combined effect of performing annealing at different temperatures and varying the lateral size of the Si ridge underlying the Ge strips is addressed by means of a synergistic experimental and theoretical analysis. Indeed, three-dimensional phase-field simulations of surface diffusion, including the contributions of both surface and elastic energy, are exploited to understand the outcomes of annealing experiments. The breakup of Ge/Si strips, due to the activation of surface diffusion at high temperature, is found to be mainly driven by surface-energy reduction, thus pointing to a Rayleigh-like instability. The residual strain is found to play a minor role, only inducing local effects at the borders of the islands and an enhancement of the instability. Published by AIP Publishing.