If we're trying to model reality as we know it, hit points just don't cut it, as we've always known them. Hit points are explicitly plot protection. (AKA luck, divine favor, dodgitude, stamina, and whatever else.) Wizards get more hit points by becoming better spell-slingers, for jiminy's sake!
Except when hit points aren't plot protection. See: sneak attack, fireballs, falling damage*, and a thousand other examples. There's no real consistency to damage or hit points, and most of us are okay with this because we're used to it. Except once in a while when one of these ideas come up, and seeing a new hole in the magician's curtain makes some of us see red.
Anyhow, if we want to model reality as we know it, we shouldn't be giving bigger creatures more hit points. That creates weird situations where bigger creatures get more skills and feats because they have more HD, and other gamist oddities. No, bigger creatures need a bit of DR, because that roughly reflects reality. A giant's skin is a bit thicker than a human's, whose is a bit thicker than a halfling's, but stab any one of them in a vital area and they're all equally likely to keel over. You don't kill an elephant with an AK-47; you kill it with an elephant gun.
If we want to model our reality, we'd need this kind of change, in addition to a dozen other changes to D&D's fundamentals.
*Falling damage in particular is proof that hit points aren't just 'meat points.' An ant can survive a fall from any height, while an elephant will get crippled or killed by a fall that would merely injure a human. Large whales don't even have to fall; they get crushed by their own weight when the poor things get beached. Funny how reality often works exactly the opposite of how D&D works.