A Fighter's body isn't really 2.5 times harder to hurt than a Wizard's, according to most theories of the game. And a 10th level Fighter's body isn't really 25 times harder to hurt than a 1st level Wizards either. He's just learned to roll with the punches, to tough it out.
A sword blow that cuts the commoner nearly in half becomes a minor bruise or shallow gash on the hardened veteran because he knows how to move, how to turn the edge or give with the blow, how to minimize damage. The sword blow was just as good in either case, but the fighter managed to turn what should have been a disemboweling strike into a mere flesh wound.
This is largely how I view it. When a character takes damage, even if it is only a single hit point, that character has suffered
some amount of 'real' damage - even if it's only a scratch. However...
A deep cut on the Wizard represents two, maybe three hit points of damage. A cut of equal severity on the fighter represents somewhere between fifty and seventy five hit points.
This isn't how I view it.
IMC, losing a greater number of hit points does
not imply that you're more seriously wounded, whether that's in absolute terms or as a percentage of the whole.
(The exception to this is if an attack kills a character
outright, in which case you can go to town on the graphic descriptions - since only very powerful magic can bring them back, it doesn't matter if they were impaled, beheaded, pulped, or whatever.)
So those 2-3 hit points on the Wizard probably
is not a deep cut, and nor is the 50-75 hit points on the Fighter. In both cases, it's just some measure of damage - and at worst, a wound that
will heal by itself if left untended.
The paradox is that the 10th level fighter is also 25 times harder to heal than the 1st level Wizard.
One is healed casually with Cure Light, while the other will take several castings of Cure Critical.
Yes, there is a problem with the healing rules in pre-4e versions of the game. Ideally, each character should have a calculated "healing value" that stands at some percentage of their total hit points, and whenever they receive healing they then gain a number of hit points calculated from that "healing value". Thus, if the Fighter has 100 hit points and the Wizard 20, a
cure light wounds might restore 25 points of damage to the Fighter and 5 to the Wizard.
Basically, that's part of the "Healing Surge" notion from 4e. And not one I really have a problem with - I didn't like the Surges themselves, but the notion of a "Surge Value" wasn't a bad one.
(There is, however, a problem with this at low levels - suddenly the 1st level Cleric can no longer provide the 1st level Fighter with an almost-complete heal with a single CLW. That has a significant impact on how many encounters the party can get through in a day, which may well be undesirable.)