Why does a heavy rifle not kick as strong as light rifle using a same bullet?

Recoil is the reaction of speed x mass of the bullet, producing recoil (reaction) consisting of speed (of the rifle moving) x mass (of the rifle). If one rifle has a greater mass than another, the recoil is trying to move the larger mass, and cannot move it as fast. This is why you can hit a Golf ball much further than you can hit a Bowling ball. Difference in mass.