From the Monster Manual intro, on p. 7, under "Hit Points":
A monster's size determines the die used to calculate its hit points, as shown in the Hit Dice by Size table.
Enlarge/Reduce gives an option to reduce the target's creature size by one level.
Reduce. The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal. This reduction decreases its size by one category – from Medium to Small, for example.
When a creature's Constitution modifier decreases, its maximum hit points decrease as well.
Following this logic, does using Reduce on a monster also reduce its maximum hit points?
Best Answer
No, it won't reduce its max HP.
This spell details exactly every change effected by the spell. Changing hit points is not listed, so it doesn't happen.
While creature size is used for initially calculating the max hit points of a new custom monster, no rule states that a creature's hit dice change when a creature changes size during play. The guidelines given in that section have no effect outside of it.