Self defense doesn't have to be proportional, there is a bit of nuance.
If you throw a punch at me and i punch you back, but you black out and injury yourself a lot as you hit the ground, it was self defense, despite the major damage. Why? Intent. You didn't want to cripple the guy, you just defended yourself and couldn't predict what would happen next. Intent, you didn't intent to cripple the guy, it was basically collateral damage.
Emotional state also matters, if you are at threat you may use more force than needed due to adrenaline.
Seeing the reaction of the black shirt guy, he didn't want to throw the guy head first at the ground, but on his back.