Thomas Shey
Legend
One's not connected to the other. It's a fair tradeoff for the differences in power between them. The d4 wizard is a glass cannon. The d12 barbarian is a meat shield. Their HP is suited to their roles in combat. You send out whatever is reasonable for the situation and let the players sort it out. If the fragile wizard wants to face-tank some giants, that's on them. If the beefy barbarian is a coward and hides behind the wizard, that's on them. The HP balance between PCs, monsters, and monster damage sorts itself out real quick and has for about 50 years.
This assumes the wizard always has anything to say about it. Actually getting anything done and avoiding, say, a group of goblin archers letting fly at them is not always something that can be done.