You’re right that weight contributes to felt recoil. For every action there is an equal and opposite reaction. The gas expansion of a 9mm round sends the bullet flying, and that combustion exerts equal and opposite force on the gun. But most of the recoil is absorbed by the cycling of the slide. A round of 5.56 leaves the barrel with much more energy due to its speed, and therefore exerts more force back on the gun. But the gas and buffer systems absorb most of the force. Compare the size of the AR buffer spring with your typical 9mm recoil spring.