Hit point inflation
Way, way back in the long-ago days of the mid-80s, I was introduced to D&D through a copy of the Moldvay Basic Rules and the accompanying module B2: The Keep on the Borderlands. Before I ever worked up the courage to ask my siblings if they might like to play this game, I had read both books cover-to-cover at least a dozen times and thumbed through the most interesting bits still more, and they left some impressions.
One of those impressions was a sense of scale regarding hit points. The average starting player character might have 7 or 8 if he were lucky, but maybe as few as 1 or 2 (though I did take to heart the suggestion that players be allowed to re-roll at 1st level only if the dice came up 1 or 2.) Lots of 1st-level NPCs had hit points below average for their class's Hit Die -- I recall plenty of Keep guards manning the walls with 3 or 4 hp. NPCs of 2nd level seemed quite tough with 7-10 hp or so, and those of higher levels with hp totals in the teens were real badasses. (I should note that Gary tended to give "important" NPCs maximum or near-maximum hp; to wit, the 3rd-level priest with 18 hp, the 3rd level bailiff with 22, and the 6th-level castellan with an astounding 48. Even being utterly new to the game as I was, that last one struck me as quite outlandish, but then, I didn't expect my players to attempt a murder spree in the Keep, so it wasn't of immediate concern anyway.)
Outside the Keep, in the wilderness and the Caves of Chaos, we had lots of 1 HD creatures like orcs and goblins, who mostly had hit points in the 3-5 range and a few 2 and 3 HD monsters with around 7-14 hit points. These latter seemed like pretty tough customers to me at the time. Then we get to the truly fearsome denizens of the caves, like the ogre (hp 25,) the owlbear (hp 30,) and the minotaur, with a whopping 35 hit points! By today's standards, those are paltry, even wimpy, but as I read them, I wondered how my players would ever defeat such terrifying foes!
Contrast this with the parallel evolution of AD&D, which was already beginning the long march toward hit point inflation with its larger Hit Dice for character classes and monsters with lots of pluses tacked onto theirs. (Basic D&D tended to go light on the pluses, with +1 most common, +2 fairly rare, and the troll with 6+3 HD as something of an anomaly, while AD&D, especially in supplements after the original Monster Manual, sported creatures with pluses equal to or greater than the number of full HD.)
I haven't taken much interest in 5e, and so haven't done a lot of research, but I've seen mention made of creatures like ogres with nearly 100 hp. It seems the mania for mountains of hit points has reached its zenith -- or perhaps its nadir -- in fifth edition.
Alas, even basic D&D ultimately succumbs to hit point inflation, as characters accumulate Hit Dice up to level 9, and monsters must keep pace. Even mighty creatures like dragons -- the red dragon as listed in the Basic Rules has 10 Hit Dice, which was mind-boggling to me at the time -- eventually become unimpressive next to a fighter with 9d8 hp plus Constitution bonuses. Frank Mentzer, of BECMI fame, addressed this problem with ... you guessed it! -- bigger dragons with even more Hit Dice and hit points.
Computer RPGs take the trend to extremes, with heroes and monsters sometimes having thousands of hit points (or health, or whatever the metric is called in any given game.) There's a certain logic to it, as big numbers allow for greater granularity with regard to all manner of bonuses, penalties, and adjustments in combat. Magnifying the hit point totals by a factor of 100 permits the use of, say, instead of a scale of damage adjustments from +1 to +3, an expanded scale of +1 to +300, with all points in between. You can have shades of variation between weapons and armor as equipment is upgraded, rather than big sudden jumps. And of course, the game engine handles all the fiddly number-crunching, so the player doesn't have to. Even then, I find those gigantic numbers a bit hard to wrap my mind around, and games such as The Elder Scrolls series wisely stick to relatively lower numbers on the player-facing side, leaving the nuances of calculation strictly behind the scenes, where they can obtain the desired granularity using tiny fractions rather than monstrous integers. (Crack open the game data in the construction set, and there are all sorts of subtle decimal values, which you never have to look at or think about in-game. The attributes you see during play top out at a relatively easy-to-grok 100, your health and magicka values at a few hundred, rather than thousands or even tens of thousands as in some other games.)
But I've veered off on a tangent. The thing is, with tabletop RPGs, it doesn't have to be this way, and it never did. A creature with 35 hit points can still be an absolute beast, whether PCs are 1st level or 10th. It's simply a matter of not making hit point accumulation the primary reward for gaining levels of experience, especially past a certain point. The authors of TSR D&D did recognize this, as evidenced by their choice to limit Hit Dice to a particular level (9th, in the case of basic) and adding 1 or 2 hp per level after that. Still, even that is excessive, in my humble opinion. There's no compelling reason not to stop Hit Dice accumulation at 3rd or 4th level (as a few games, such as The Hero's Journey, do.) Characters still become more formidable in terms of fighting ability on the attack tables, thief skills, and spell progression; they just don't get to have as many hit points as a dragon or elephant.
With more stringent limitations of hp for characters, both PC and NPC, monsters in tabletop games don't need to have utterly massive hit point totals to be tough and dangerous. No likely opponent in B2 had more than low double-digit hp. This appeals to me, not least because the math is so table-friendly and non-intimidating. I have a pretty good head for mathematics, but I don't really relish grappling with big numbers in the heat of a good game session, as one must do if every opponent is a mountain of hit points.
It's more than a math thing, though. Letting large powerful creatures like dragons and elephants always have more hp than characters helps to maintain a sense of awe at really big, tough monsters. You don't have to trivialize them by throwing even bigger, tougher opponents at higher-level PCs, and in the process turning combats into ever more boring slogs of attrition against ever-larger piles of hp. You don't need to bolt on subsystems or dole out scads of magic items to allow PCs to inflict increasingly large amounts of damage to mitigate the slog and further trivialize elephants and dragons. There's no good reason for PCs ever to be on an equal basis with such opponents, and the game would be a lot better for it.