My character's arms were stretching despite having Arm IK. I tried applying the Arm IK behaviour Arm ID to the character, the body, even the arms. I was testing whether it was working by trying to stretch the arms.
It still wouldn't work. I watched the tutorial several times and triple checked the rigging. Finally, more by luck than good management, a solution.
I had been testing by dragging the hands. When I dragged by the wrists, everything worked and the arms would only stretch to the expected 10% default.