For the same gun and the same charge, consider the extremes: a bullet
that is much much less massive than the gun and a bullet that is much
much more massive than the gun. In the former case almost all of that
energy from the charge that contributes to the macroscopic kinetic
energy of the bullet and the gun, is transferred to the bullet. In the
latter case almost all of said energy is transferred to the gun. The
more macroscopic translational kinetic energy the gun has, the greater
the recoil. In the continuum connecting the extremes, the more massive
the bullet, the greater the fraction of the energy from the charge the
gun gets, hence, the greater the recoil.