Despite extensive studies on colloid transport in porous media, the influence of flow velocity on the size exclusion effect (SEE) remains poorly understood. In this study, nuclear magnetic resonance (NMR) and micro-computed tomography (μ-CT) were employed to characterize the pore structures, and column experiments combined with HYDRUS-1D modeling were conducted to quantitatively analyze colloid transport under varying flow velocities. NMR and μ-CT results indicated that coarser sand possesses a larger average pore diameter. Colloids consistently broke through earlier than the conservative tracer, confirming SEE. Both (Δ<em>T</em><sub>b</sub>) and the relative peak arrival time (Δ<em>T</em><sub>peak</sub>) decreased with increasing flow velocity, indicating that SEE weakened as flow increased. Across all media, γ declined from about 0.09–0.14 at low velocity (~0.004 cm s<sup>−1</sup>) to about 0.03–0.04 at high velocity (~0.03 cm s<sup>−1</sup>). In addition, γ was generally higher in 10 cm columns than in 30 cm columns, especially at low velocity. Flow simulations further suggested that increasing velocity reduced the dominance of low-velocity regions and enhanced the continuity of active flow pathways. These results indicate that γ is not a fixed geometric constant, but a flow-dependent effective parameter governed by pore accessibility and transport conditions.