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?