It is more nuanced than that. I does not necessarily have to be equal to the threat, but must be the minimum means required to stop the attack that is available to you at the time.

So if someone runs at you with a knife and all you have is a gun, you're not expected to drop the gun and start looking for a knife to defend yourself with. But if someone who is half your size comes at your bare handed, you shouldn't need to do much more than use measured open handed techniques like pushing them away. Someone this much smaller isn't much of a threat and there is no need to pummel that person into a paste to stop their attract. Your actions shouldb also immediately stop as soon as the threat is no longer active and you absolutely shouldn't be allowed to use it as an excuse to cause unnecessary harm.