For 3D printed concrete (3DPC) to be a valid alternative to traditionally cast concrete, the technology should be able to exploit its advantages such as full production process automation and the realization of complex geometries. With these complex geometries, the opportunity arises to start designing more efficient and more optimal structures, thereby allowing to reduce the amount of material used in structural elements. Ensuring the structural capacity and providing sufficient ductility often entails the introduction of reinforcement, which is far from evident for current additive manufacturing processes. To solve this issue, this work presents a hybrid 3DPC-cast beam approach, where the lower chord of the beam consists of traditionally reinforced cast concrete. The upper, and most critical, part consists of an unreinforced 3DPC segment, for which the volume is reduced through both a simplified approach and a structural optimization method. A design situation is considered where the beam is subjected to a simply supported bending scenario. For both volume reduction approaches, a range of residual volumes and reinforcement ratios are implemented, after which critical failure modes and corresponding structural capacities are discussed based on analytical calculations. The imperfect bond between the printed and cast concrete is considered by enforcing a maximum interfacial shear strength. A distinction is made between configurations where the hybrid beam fails as a whole, versus configurations where at first the interface between the printed and cast concrete fails and where ultimate failure is characterized by the failure of either the printed or cast component. Constant height and constant capacity solutions are discussed, together with their structural implications. The results highlight the trade-off that is made between more sustainable, volume-reduced solutions, and the reduced capacity or increased beam height that accompanies these more complex solutions.