An 8k heal on a guardian with 11k hp and 3200 armor is much more significant than an 8k heal on a warrior with 20k hp and 2200 armor.
No, it's not. It's only significant in that its a higher percentage of your health pool, but that's meaningless. Guardians do have more sources of healing than warriors do, though. And each one of those additional heals that you have and a warrior doesn't are what mitigate the perceived HP gap. Warriors don't have Altruistic Healing, which goes a very long way in closing the gap.