Maximum dust-to-gas volume density ratio (ρ_d/ρ_g)max as a function of time after the dust particle initialisation t – t_d,init. In the left, middle, and right panels, respectively, simulations of the streaming instability only, of the scenario SIwhileVSI, and of the scenario SIafterVSI with different initial dust-to-gas surface density ratios (Σ_d/Σ_g)_init, fixed dust sizes a or Stokes numbers St, and resolutions (fiducial or double) can be seen. Increasing any of the three parameters causes the maximum volume density ratio to increase as well. In addition, the maximum is larger if a dust size of a = 3 cm rather than a Stokes number of 0.1 is considered, as is evident from the left and middle panels. Neither the simulation of the streaming instability with the doubled resolution nor the corresponding one with the fiducial resolution develop into a strong-clumping phase, with the maximum volume density ratio being greater by a factor of a few in the former simulation. In comparison, the maximum is enhanced by more than an order of magnitude during the strong-clumping phase in the double-resolution simulation of SIafterVSI than in respective fiducial-resolution simulation.