If hit points are abstract, increasing hit points with level advancement represents increased work that needs to be done by enemies/traps/nature to inflict the same amount of damage on a PC, not increased damage that a PC can take. If that's true, why does it take more healing to heal the same proportional loss of hit points to a higher level PC that has more hit points?
Dave Hargrave points this out in his third Arduin Grimoire, asking "why is it that a tenth level character requires several Heal Lesser Wounds rituals to fix up the same proportional damage that a first level character needs only one lesser heal to fix? The disparity gets greater the higher in experience that a character gets."
He then goes on to suggest a four-tiered set of Clerical healing spells to replace the default model:
Heal Lesser Wounds heals up to 25% of the character-being-healed's hit points. Heal Wounds of the More Serious Sort (seriously, that's what he calls it- not "Heal Moderate Wounds" or anything short like that- that's the Arduin Grimoire for ya) heals up to 50% of a character's hit points. Heal Grievous Wounds heals up to 75% of a character's hit points. Heal Critical Wounds heals up to 100% of a character's hit points.
As a ref who's running an Old School game but has some players who are inclined to New School (3.5) gaming and are playing not because it is an Old School game but because it's gaming with friends, I'm always on the lookout for more ways to sell Old School gaming. I'll probably be introducing these mechanics into my game the next time we game as a way to cement hit points as an abstract concept. I don't currently allow clerics, but the box of super-powerful healing pills the PCs have will either be Heal Grievous Wounds or Heal Critical Wounds. We'll see what they think.