The study aims to investigate the influence of maximum grain size of coarse aggregate on the deformation of hydraulic concrete, and assess whether the aggregate sizes specified by national standards meet the performance requirements for hydraulic concrete. Based on the Yangtze-Huaihe River Diversion Project, we conducted deformation tests on concrete with maximum coarse aggregate sizes of 31.5 mm and 40.0 mm and examined the evolution law of long-term durability performance of both aggregate sizes. Results indicate that concrete with a maximum aggregate size of 31.5 mm exhibits slightly higher ultimate tensile strength and modulus of elasticity, along with improved dynamic fatigue performance. In contrast, concrete with a maximum aggregate size of 40 mm shows reduced drying shrinkage and autogenous volumetric deformation, with comparable creep behavior to that of concrete with a 31.5 mm aggregate size. However, concrete with a 31.5 mm aggregate size demonstrates superior long-term impermeability and frost resistance compared to concrete with a 40 mm aggregate size. Therefore, using a maximum aggregate size of 31.5 mm in hydraulic concrete not only meets the performance requirements but also enhances dynamic fatigue life and durability. Nonetheless, attention should be given to potential concrete cracking issues.