Note that most of the published monsters are close to this system, but not exact, as they were likely tweaked in playtesting. The relevant table is on page 274 of the DMG, but the important parts are to the right. And maxing out a monster’s HP could functionally double it, which could push it multiple CRs higher in some cases. Even a monster with extremely low HP might obliterate a lower-level party if it wins initiative (especially if it also has a high AC), since damage scales so high. This means, if you try to adjust the monster’s HP, your intuition might be wrong for how much that lowers or raises the difficulty. The designer was trying to hit a certain HP total, and adjusted the number of HD until the average value of those dice plus Con mod got into the right range. The hit dice for a monster in 5e are basically completely arbitrary. If you’ve come from previous editions where the total number of HD was equivalent to the monster’s level, that is no longer the case. If you haven’t looked into the monster math for 5e (starting on page 273 of the DMG), your intuitions about altering HP might be wrong. Meanwhile, GMs often figure that if they just set a monster higher in its HP range (by default, they take the average dice rolls) that’s still fair but might make the fight a little more challenging. One of the classic ways to put a higher-difficulty monster into a module for lower-level characters is to have it be already injured before they fight it, giving it fewer HP.

D&d 5e monster