Hit Points aren't the same thing as damage, though, even when you're talking about healing. You could have damage be purely physical, while HP - your capacity to sustain damage without dying - grow by luck and skill and whatnot.
The simplest and most consistent model (though not necessarily the best, depending on personal priorities) is just that 5 damage is 5 damage, regardless of who takes it - it's a direct impact of an arrow upon your armor, converting a defined amount of force into a proportional amount of blunt trauma. Then, the difference between a squishy thief and a stout paladin is the number of those hits that they can take before falling. The thief, being a chump, is knocked out from the second impact; the paladin, being generally tougher and also more experienced at taking a hit, can stay conscious through several such impacts.
Although, from what I recall about 2E, your Constitution score wasn't a factor in recovery time - you always healed just one or two points per day, and Con only factored into how many hits you could take. If it took the paladin twice as long to heal as the thief, then it was only because the paladin had suffered many more injuries, such that equivalent trauma to the thief would have been fatal.