The concrete enlarged-diameter piles, as a novel type of variable-section pile foundation, exhibit significantly higher bearing capacity than conventional straight-bore piles. To systematically investigate the influence of flange number on vertical compressive bearing capacity, this study adopts a combined approach integrating laboratory model tests and numerical simulations. Finite difference method (FDM) models and coupled finite difference–discrete element method (FDM–DEM) models were established for straight-bore piles, single-plate concrete expansion piles, and double-plate concrete expansion piles, enabling the mechanical behavior of the pile–soil system to be analyzed from both macroscopic and microscopic perspectives. The results show that the numerical simulation outcomes obtained from both modelling approaches are in excellent agreement with the laboratory test results, thereby confirming the reliability and accuracy of the simulations. The enlarged-head configuration exhibits a pronounced enhancement effect on the bearing capacity of pile foundations. Specifically, the ultimate bearing capacities of single-plate concrete expansion pile and double- plate concrete expansion pile reach 1.67 and 2.29 times that of straight-bore piles, respectively, demonstrating the superior strengthening effect of the double-plate design. With increasing vertical load applied at the pile head, the strong contact force chains beneath the enlarged-head structure become increasingly dense, indicating that the enlarged-head structure effectively mobilizes a greater number of surrounding soil particles to participate in load transfer. Simultaneously, soil particles beneath the enlarged-head exhibit distinct downward and lateral compaction displacements, eventually forming a characteristic heart-shaped slip surface beneath the enlarged-head. In conclusion, the coupled FDM–DEM model accurately captures the complex interaction mechanisms between the continuum pile structure and discrete soil particles. This modelling framework provides reliable technical support for investigating pile–soil interaction characteristics and for the quantitative analysis of the soil reinforcement zone surrounding enlarged-head piles.