The stress–strain curves of A356 cast aluminum alloys exhibit an unusual size effect on flow properties: the finer the microstructure, the lower the tensile flow strength. Tensile tests were carried out on specimens made of an A356 alloy with 7% Si as the main alloying element. The specimens were cast at two cooling rates. For both processing conditions the microstructure within each grain consists of pro-eutectic aluminum dendrites separated by a boundary eutectic region of segregated silicon particles of ≈2–3 µm diameter. The fast cooling rate gives rise to a secondary dendrite arm spacing of approximately 20–30 µm, while the secondary dendrite arm spacing obtained with the slow cooling rate is about 80–100 µm. Discrete dislocation plasticity is used to model the inverse size effect in this alloy. The dislocations are represented as line defects in an elastic solid and dislocation nucleation, annihilation and drag are incorporated through a set of constitutive rules. Obstacles to dislocation motion are randomly distributed in the dendrite and the eutectic regions, but with different densities and strengths. The thickness of the eutectic region is found to be a key parameter in determining the inverse size effect. In addition, the size effect is found to depend on the extent to which dislocation nucleation takes place in the eutectic region.