What happens to a creature who is enlarged and then reduced?

Question

Inspired by Does an Enlarge/Reduce spell on a druid affect the beast form if the druid Wild-Shapes?

Focusing on the specific effect of the Enlarge/Reduce spell:

You cause a creature or an object you can see within range to grow larger or smaller for the duration.

Consider a human who is, for reasons which will become obvious, named Inflatoman.

Inflatoman is the target of an Enlarge spell cast by Engorgo. He grows to twice his size becoming a large creature.

While the Engorgo maintains concentration; Reducto hits Inflatoman with a reduce spell, which clearly causes him to revert to his original size. While these spells have not cancelled each other, their effects have.

So, what happens:

if Engorgo stops concentrating while Reducto keeps concentrating:

Inflatoman stays his normal size; Reducto's spell has already done all the reducing it can do.
Inflatoman shrinks to small size; with Engorgo's spell removed the effect of Reducto's reasserts itself.

if Reducto stops concentrating while Engorgo keeps concentrating:

Inflatoman stays his normal size; Engorgo's spell has already done all the enlarging it can do.
Inflatoman grows to large size; with Reducto's spell removed the effect of Engorgo's reasserts itself.

An alternative way of asking this is: Is the growth from an Enlarge spell a one shot effect that causes the target to grow at the beginning and then shrink at the end OR a continuous effect that is trying to make the subject be twice the size they would otherwise be? Vice versa for Reduce.

jkat718 · Answer

From the "Combining Magical Effects" section of the Player's Handbook (p. 205) and the Basic Rules (p. 85):

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.

This rule can lead to one of two different rulings:

The DM could rule that, since Enlarge/Reduce is a single spell, the more recent casting of it (Reducto's) would take precedence, and Inflatoman would not only return to normal size but actually become Small.
The DM could rule that Enlarge and Reduce — while the same spell — are considered separate for the purpose of combining effects, and rule that the two effects cancel out, resulting in a Medium-sized Reducto.

If Engorgo were to stop concentrating, then Inflatoman would either remain Small (option 1) or become Small (option 2). If Reducto were to stop concentrating, then Inflatoman would become Large in either case.
Thank you to @V2Blast for notifying me of the 2018 PHB errata on this!

xanderh · Answer

When the spell ends (for whatever reason), their effect ends too. Enlarging a creature makes it grow when the spell is cast, and shrink when the spell ends.

From this, we can conclude that, no matter your interpretation of whether the spell is maintaining a certain size for the creature or not, when Engorgo stops concentrating on his spell, Inflatoman shrinks one size category, and when Reducto stops concentrating on his spell, Inflatoman will grow one size category.

Adeptus · Answer

Given that the spells require concentration, they are an ongoing effect. 
When the caster's concentration ends, the ongoing effect ends.

To compare against your alternative, let's look at what happens:

The spell is an ongoing effect that lasts while concentrating.

Caster casts spell. Target's size is changed.
Caster stops concentrating. Ongoing effect ends, returning target to normal.

The spell is an instantaneous effect.

Caster casts spell. Target's size is changed.
Caster stops concentrating. A secondary instantaneous effect is triggered, returning target to normal.

What happens to a creature who is enlarged and then reduced?

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