Only buffing HP wouldn't do it. By adding 2 more members in your party of 4, you also multiply the potential of your party by a lot of factors. You have more CC potential, AoE targeting allies become way more powerful, you can get more people into melee range making spellcasters more safe, you can further abuse synergies like surfaces, you can cover a wider range of damage types... All those factors makes it impossible to just add a flat multiplier and be done with it.
