Bone rotation bends and scales mesh instead of rotating it?

I bought this mesh that I am trying to rig. The torso is kind of wide and I put a bone right down the middle. I have all its vertices 100% weight painted to the bone. When I rotate the bone, the torso rotates with it, but as you go farther from the center of the bone, the object bends and scales down as well. So it curves and shrinks in addition to rotating.

I’ve already normalized the object’s scale so it’s at 1,1,1 but I don’t know what else I can do to fix this.