<p>Aggregate content plays a critical role in governing both the rheological behavior and mechanical performance of 3D printed concrete (3DPC). This study investigates the impact of varying the aggregate-to-binder (A/B) ratio from 0.5 to 1.5, while maintaining a constant water-to-binder ratio, on the printability, mechanical behavior, and volumetric stability of 3DPC. A dual-parameter framework based on flowability and shape retention is proposed to define printability across different A/B ratios, offering a practical and generalizable alternative to rheometer-based evaluation. The experimental results demonstrate that the proposed dual-parameter framework effectively enables rapid evaluation of 3DPC printability. When both flowability and shape retention are maintained within appropriate ranges, 3DPC exhibits good printing performance. Mechanical testing reveals that increasing the A/B ratio above 1.25 reduces both compressive and flexural strength due to weakened matrix–aggregate bonding and interfacial failure. Furthermore, anisotropy in strength distribution emerges at extreme A/B values due to variable interlayer cohesion. Drying shrinkage is significantly reduced at higher A/B ratios, with a threshold of A/B ≥ 1.0 ensuring compliance with ACI volumetric stability standards. Notably, the binder contributes approximately 89% of the total CO₂ emissions associated with 3DPC. Consequently, the overall carbon emissions decrease markedly as the A/B ratio increases. An optimal A/B ratio of 1.25 is recommended for designing 3DPC, as it provides a well-balanced performance across mechanical strength, volumetric stability, and environmental sustainability.</p>