We usually talk about HP bloat as a modern problem in D&D; something that started (or at least accelerated) in 3e and peaked around 4e and 5e. To sum it up, the idea is that a 9th-level fighter might have 50-60 HP in B/X, but more likely about 80-100 in modern D&D.
(I'm using B/X and 5e as my examples here because these are my favorite versions of D&D. The comparison is not perfectly suited for other editions. AD&D has bigger numbers than B/X overall, and BECMI goes all the way to level 36, while 4e goes to level 30)
Some people will even notice how modern PCs got it easier since they have more HP. In the old days, even a goblin could drop you with a single hit!
Well, this second part is just wrong. A 5e fighter has about 12 HP while a B/X fighter might have half as much. But while a B/X goblin might deal 1d6 damage, the 5e goblin deals 1d6+2 damage; it also has twice as many HP and can hide with a bonus action, which mean it is likely to attack you with advantage, increasing the chances of a hit (and of a critical hit). It's attack bonus is +4 (instead o ZERO), which you usually make PCs get hit more often than they did in B/X.
(As an aside, if you allow AD&D fighters extra attacks against HD 1-1 creatures, a 9th level fighter might have a BETTER chance against nine goblins than a 5e fighter!)
So, maybe the problem is not HP bloat, but everything bloat; HP is higher, but damage is also higher, and more powers are available. In 3e (and even more in 4e), attack bonuses and defenses were also a lot higher when compared with old school games. The same applies to skill bonuses, saving throws, and so on.
(This might be a problem when the inflation becomes REDUNDANT; say, by doubling all HP and ALSO all damage, or by giving ludicrous skill bonus but also enormous DCs. The advantage is that this gives more granularity; maybe too much).
5e dialed it back almost everywhere except HP and damage. A 9th level fighter might have a +8 or +9 attack bonus, while a B/X fighter has +5 (+7 on level 10). [This is part of what they call "bounded accuracy, BTW]
We could think of it this way: a fighter has to be level 7 on B/X to get that +5 bonus. At this point, he has 7d8 HP at least (it is likely that he has a bit more due to Con bonus; let's say about 35-40 HP). In comparison, a 5e fighter STARTS with a +5 attack bonus (and about 12 HP). This is unlikely to change until level 3 (which means about 24 HP).
In short... if you compare fighters with the same attack bonus, the old school fighter has more HP!
In other words, I think the right question is not "how many HP a 7th level fighter has in B/X when compared to 3e (or 4e, 5e)", but "what level do you have to be in 5e to be comparable with a 7th level fighter in B/X"?
There is not a perfect answer, but I'm guessing it would be about 3 or 4. Ultimately, this is comparing oranges and apples. But we can certainly say that a 7th level fighter in B/X has more in common with a 4th level fighter in 5e than a 7th level fighter in 5e!
Of course, this changes when you reach level 9; in old school games, your HP will raise slower after that. However, since you gain levels a lot faster, the amount of HP per XP is raised too!
Anyway... modern D&D does not exactly inflates HP; what does is that it takes PCs to a higher magnitude of power.
For example, while in B/X you would reach (at most) level 14th with (at least) a +9 attack bonus and (at least) 9d8+10 HP, in 5e you can do the same by level 9, and you still have 11 more levels to go.
Now, I'm not saying the modern method is better; in fact, I usually stop my campaigns by level 10 when playing 5e, and even published campaigns usually stop around 10-13th. The "additional tiers" of play that modern D&D adds are a lot less useful for me than each of the 14 levels you can get in B/X. My own game stops at level 10 (which is roughly comparable with level 14th in B/X). Because of that, material for higher levels are mostly "filler" for me - but they might be useful and fun for other players.
In short, "HP bloat" is not really a thing that can be separated from "power bloat" (or, at least, "numbers bloat"). The important part is not how many HP you've got, but what kind of heroes (or superheroes) you want to play.