High-pressure die-cast (HPDC) magnesium (Mg) and aluminum alloys enable vehicle lightweighting while reducing manufacturing costs by simplifying part assembly. The increasing use of super-large castings in electric vehicles enhances structural reliability and cost efficiency. However, HPDC Mg alloys face challenges related to casting defects such as porosity, cold shuts, and oxides. These defects influence tensile strength and ductility, depending on their location and size. This study employs finite element (FE) modeling to investigate how a dominant large pore, its position, and the sample size affect the ductility of thin-walled HPDC Mg. Motivated by the ductility variations reported in literature and the experimental findings on AM60 castings, synthetic microstructure-based models are used to assess the effects of different pore sizes and locations. The results indicate the presence of three different regions based on the large pore size and model size: 1) a region dominated by the effects of the large pore, 2) a plateau region dominated by pore interactions, and 3) a transient region between these two effects. A threshold distance from the sample edge ( $\mathrm{d}\approx 0.9\sqrt{\mathrm{D}\cdot \mathrm{L}}$) is proposed, within which a large pore can significantly reduce ductility. Additionally, large pores near edges contribute to ductility variations in Mg castings.