Telling the players this type of information sounds like really playing the numbers to me.
If that's the way you want to run it, go ahead. It just doesn't sound particularly fun or useful to me. As a DM, I let the players know, because their PC's know how the world and their abilities work.
It's reasonable for the PC to know what it looks like when someone successfully saves against their fireball as opposed to taking it full in the face and just shrugging off the damage. Or when they hit something with a solid sword blow and it just doesn't do the damage you would expect. Hit points are an abstraction, so using them to gauge the strength of a hit isn't that useful. That's what the DM is for - to use narrative and description to let the players know how the creature is reacting to the attack.
A creature that makes the save and evades some of the damage reacts differently than if they fail the save and resist the damage - one is maneuvering so that the spell doesn't impact them fully, the other just shrugs off part of the damage. They take the same damage, but they react to it differently.
A creature that both has resistance and makes the save would take far less damage than expected.
The actual hit point totals are almost irrelevant (other than a 50 hit point creature being hit by a fireball appears much more injured than a 500 hit point creature, regardless of the actual damage dealt). Telling the PC that 500 HP creature took the full brunt of the spell and seems only minorly injured is a narrative way of saying 'This thing has a ton of HP, buckle up."
I don't see value in withholding that information from the PC's. It's just not fun for the players.