<p>Shock compaction of granular materials has been of significant interest to industry and defense sectors. This research investigates the influence of particle size—one of the key characteristics of granular materials—on the response to shock compaction through mesoscale computational modeling. Simulations evaluate three different particle sizes for circular and elliptical grain sets in ideal packing arrangements depicting two extreme values of the porosity measure, specifically quadrilateral packing (QP) and hexagonal packing (HP). Results were analyzed for interrelated particle-scale responses, specifically local porosity, particle strain, and pressure, highlighting their dependence on particle size during shock compaction. Further, under favorable conditions, results showed a delay phenomenon that led to a significantly slower shock wave velocity in QP, while HP maintained constant velocities regardless of particle sizes. Simulations indicate a delay in shock wave propagation attributed to <i>dynamic force chain buckling</i>. While the force chain buckling phenomenon has been commonly reported primarily under quasistatic or stress-equilibrated conditions, it has not been identified under shock loading or stress-unequilibrated conditions. This work seeks an improved understanding of the instability response to model this newly identified energy dissipation mechanism under weak shock.</p>