I somewhere saw people discussing the question whether the thief in B/X would be better having a d6 for hit dice instead of a d4, and one point that was brought up against that is that the thief gains new levels very quickly and as such has more Hit Dice than other characters with comparable XP, and that this would even things out already.
I’ve long felt that the speed of thieves gaining new levels has been greatly overstated, and so I went to check how the average hit points over time for the different classes actually look like.
As can be seen here, a thief’s average hit points do get very close to those of a cleric on reaching 4th and 5th level, but the cleric almost immediately surges ahead again.
And after those initial first levels, the gap between thieves and clerics only widens until 10th level when the cleric continues to gain only 1 hp each time compared to the thief’s 2 hp. Now a 6 hit point gap on average towards the end of the B/X progression is not that big, but at these small numbers that’s still +20% for the cleric. And a +50% for the fighter, compared to the thief.
Does that make thieves too fragile at higher levels? I don’t know, I’ve not enough experience at play at those levels. But I think this also shows that the thief’s faster level advancement rate does not negate the difference in Hit Die size to the cleric at all.