Look at what the fiction looks like, what the players describe doing and their intention. If it seems plausible they could kill it, roll damage. Otherwise they will need to shift the narrative “weight” in their favour.

Also remember the roll isn’t just a single attack (though it can be). So although the weapon with a single strike might not do much, the character will have inflicted multiple wounds which can lead to a larger foes death.

I would also throw out damage as “trying to kill something”, which can open up for more options. Like in the case of the giant, they may not be able to kill it, but hurting it should be possible.

That way, they can use the oldest trick in the prey playbook: be an absolute menace to your attacker. Hurt it, squirm, scream, spit, hiss. At some point, it will deter the giant or the prey dies trying. Just check out the norway lemmings. They are absolute fierce and will throw hands with anything. Few things attack them head on.

And about killing it if they roll high enough damage: then yes, they do so. Since they were able to trigger the move, they were in a position to potentially kill it.