Using Giant Slayer punishes you for being ahead and rewards you for being behind.
The basic purpose of Giant Slayer is that it's supposed to be an anti health stacking item that isn't appealing on anyone that build's health themselves, right? The bonus damage is determined by health difference. Basically it scales negatively with your own max health, but every champion gains HP per level. The higher your level advantage, the more base HP you'll have while your opponents will have less, this makes the item somewhat rubberbandy as when two champions that didn't build health go against each other it gets stronger the further behind you are, and weaker the further ahead you are.
EDIT: My analogy was bad, a better example would be fighting a tank in general while you have a lead.