The soil gas diffusion coefficient (Dp) and air permeability (ka) and their dependency on soil air content (ε) control gas diffusion and advection in soils. This study investigated the effects of average particle size (D50) and dry bulk density (ρb) on Dp and ka for six sandy soils under variably saturated conditions. Data showed that particle size markedly affects the effective diameter of the drained pores active in leading gas through the sample at −100 cm H2O of soil water matric potential (calculated from Dp and ka) as well as the average pore diameter at half saturation (calculated from the water retention curve), both exhibiting similar and exponential relationships with D50. Under variably saturated conditions, higher Dp and ka in coarser sand (larger D50) were observed due to rapid gas diffusion and advection through the less tortuous large-pore networks. In addition, soil compaction (larger ρb) simultaneously caused reduced water blockage effects and a reduction of large-pore space, resulting in higher Dp(ε) but lower ka(ε). Two recent models for Dp(ε) and ka(ε) were evaluated: the water-induced linear reduction (WLR) model for Dp, and the reference-point power law (RPL) model for ka, with reference point ka set at −100 cm H2O. The performance of both models for the sandy soils (particle size range 0.02–0.9 mm) was improved if the pore connectivity–tortuosity factor and water blockage factors were assumed to be functions of D50 and ρb. Water blockage factors, N for the WLR Dp(ε) model and M for the RPL ka(ε) model, showed a strong nonlinear relationship (R2 = 0.95) that seems promising for predicting Dp(ε) from the more easily measureable ka(ε).