The magnetic response of dynamic modulus was investigated for polyurethane-based magnetic elastomers densely packed with magnetic particles with different diameters (7 μm and 235 μm). The density indicated that voids were created at volume fractions of magnetic particles above 0.47 (87 wt%) for 7 μm and 0.44 (85 wt%) for 235 μm. At volume fractions below these critical volume fractions, no apparent increase in the storage modulus was observed for the magnetic elastomers by applying a magnetic field of 500 mT. At above the critical volume fractions, dramatic increase in the storage modulus was observed; the maximum changes in the storage modulus were 8.0 MPa and 6.0 MPa, which corresponds to the relative changes in the modulus (ΔG/G0) of 74% and 11%, for magnetic elastomers of 7 μm and 235 μm, respectively. These results strongly indicate that the creation of a space enables the movement of magnetic particles in the elastomer resulting in the formation of chain structure. It was also found that the creation of a space in the matrix of elastomers can be detected by the critical strain, the amplitude of Payne effect, or the critical magnetic field since significant changes in these parameters were found at the critical volume fraction for both magnetic elastomers. SEM images displayed a clear difference in the creation process of voids that magnetic particles of 7 μm suddenly formed many macroscopic voids at the critical volume fraction, meanwhile, magnetic particles of 235 μm formed gaps around magnetic particles. The storage modulus for both magnetic elastomers changed perfectly in response to the magnetic field even after the on-off switching of the magnetic field with 20 cycles.