Mhm, I've actually done some reading around and heres are a few associations when its okay to have super high hp. This is in no way all of them, but the idea should get across.
When the player has lots of defense attributes, when you have 30% fire protection and 60% pierce protection you need high health numbers to get a precise damage amount.
The player needs to slowly lose health, ever play metroid prime 2? If the health units were 5 each, and you slowly lost hp, at the same ratio that you would normally lose it when in a toxic enviornment, then it would not be as tense. Because losing 1 health outa 5 in 2 seconds feels different than losing 20 outta 100. Another way to justify that is of health is that they have constant situations where they subtract 1 health unit at a time.
In cave story you usually just get hit and then lose a chunk of your maximum hp. There is typically no need to get into more finite detail. You could just multiply everything by 2; damage, Max health, exp gain, etc... But then it would be exactly the same, just with bigger numbers. A rule I typically follow is if it doesn't serve a purpose for the game, then don't include it.