[MegaBlues]

What does 600 damage to one person might do 50 damage to someone else.

SL2 is a game that, despite its apparent lack of variety, has a giant amount of variance due to strategies, items, stats, etc etc. That's why most attempts of giving examples based off of what's "average" get written off because the idea of average varies from person to person.

To fix this, you would need to reduce variance (addition not multiplication), agree on what an average is, and adjust to move everything closer to this average.

all of which would require lots of heartache, argument, and effort, so you'd better just get used to playing the game where everything is so strong it's balanced and if you're not up to snuff you get stuffed

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But for a direct answer, I think that adding another multiplier would only exacerbate the issues. Person A has 300 HP, Person B has 500 HP. Assuming they deal equal amounts of damage to each other, doubling their HP would only make Person B twice as hard to kill, only making the problem worse in the end. The only way this possibly benefits the person with lower HP is if they gain enough to be able to survive one whole turn instead of being instantly killed, and then for them to be able to use this single turn to make a significant change to the battle flow.