Abstract: The significant discrepancy between the calculated deformation values based on the traditional large-scale triaxial test results on rockfill materials (with the maximum particle size of 60 mm) and the monitoring data from the prototype dam raises concerns about the factors causing this difference and their respective influences. The scale effects (including scaling methods and particle size effects) of the mechanical properties of rockfill materials in five world-class high dams are investigated using the first super large-scale triaxial apparatus in China (with the maximum particle sizes of 200 mm and 160 mm) as well as a large-scale triaxial apparatus. The influences of the constitutive models (such as the Duncan E-B and E-μ models, and generalized plasticity model) on the calculation of dam deformations are examined. Additionally, by incorporating the monitoring data from Lianghekou and Aertashi dams, the rules of the scale effects and the rationality of constitutive models are explored, thereby clarifying that the errors related to the scaling methods, particle size effects and constitutive models are the primary causes of distortion in the calculations of high rockfill dams. The research findings indicate that under the same void ratio, the mixed-scale method tends to overestimate the modulus of rockfill materials compared to the parallel grading method. Moreover, the traditional large-scale triaxial tests based on the parallel grading method tend to overestimate the modulus of rockfill materials compared to the results obtained from the super large-scale triaxial tests, which effectively eliminate the size effects. Furthermore, the calculations based on the generalized plasticity model, particularly in terms of horizontal displacement, are more in line with the actual observations than those based on the Duncan E-B and E-μ models. Finally, a scale effect classification correction method for model parameters is proposed specifically for the Duncan E-B and E-μ models. The research outcomes contribute to a deeper understanding of the scale effects of rockfill materials and the influences of different constitutive models. They also provide reliable experimental and numerical calculation references for the deformation prediction in similar engineering projects.