It's not the weight of the bullet.. it's the velocity of the bullet against a person who is standing still.
Inertia:
"Inertia is the resistance of any physical object to any change in its state of motion, including changes to its speed and direction. It is the tendency of objects to keep moving in a straight line at constant velocity. ...Thus, an object will continue moving at its current velocity until some force causes its speed or direction to change." (bbm)
The velocity of the bullet was not enough to make MB fall back as he was moving forward (charging), thus his body would stay forward in the same motion. If he were standing still, no motion, no movement, no velocity, the body would fall backward.