Actually that's conservation of momentum.
Momentum is mass * velocity. For a given weight and velocity of shot the heavier the gun the lower it's opposite velocity will be. Of course it's not quite that simple, the force is indeed mass * acceleration, but acceleration is affected by the recoil pad, the way your particular shoulder "gives" and a host of other factors. Still, in general a heavier gun will produce less felt recoil for a given weight and velocity of projectile(s).
Perhaps of interest,
My understanding is that automatics have a lower "perceived" recoil, mostly because the recoil is spread over a slightly longer time. As the bolt operates it moderates the rearward movement of the shotgun, so instead of one big shove you essentially get a shove, a drop in speed, then a smaller shove, all in a few hundredths of a second.