The bearing mechanisms and pile group interaction behavior of variable cross section piles (VCSPs), as compared to conventional uniform cross section piles (UCSPs), remain insufficiently understood. This study elucidated the underlying mechanisms from the single pile to the pile group level through laboratory model tests and finite-element analyses. At the single pile level, results indicated that the ultimate bearing capacity of VCSPs was significantly enhanced relative to UCSPs and exhibited a positive correlation with the number of variable cross section (VCS) rings. Particle image velocimetry analysis revealed a distinct transition in soil failure mode: UCSPs failed predominantly by tip-driven penetration, whereas the failure mode in VCSPs was independent of the specific configuration of the rings but governed primarily by their diameter and ring spacing. Specifically, failure transitions from localized shear around individual rings (ring spacing >3.5D_v) to global shear failure (ring spacing <1.85D_v). At the pile group level, the interaction factor α for VCSPs was consistently higher than that for UCSPs and increased with the VCS ratio (D_v/D_p). Furthermore, α grew with the pile–soil modulus ratio (K), though the rate of increase diminished beyond K > 500. In addition, α decreased with rising slenderness ratio (L/D_p) and spacing to pile diameter ratio (S/D_p).