Bending and rotating limbs doesn't mean like, bending the limb part of the limb, but bending at the joint. When you bend your arm, your bones don't break, your forearm just pivots (rotates) around the elbow.
As you have it, Mr. Mega doesn't seem to be bouncing in a humanly fashion so much as bouncing in a rubbery fashion, squashing downward rather than bending his knees.
You've added weight to his whole self, it seems, but didn't add any weight to the massivest part: the gun.