How does the Enlarge/Reduce spell work to enlarge a creature when the target creature is surrounded?

Question

I'm trying to understand how enlarging a creature works when said creature is surrounded.

According to the description of the enlarge/reduce spell:

If there isn't enough room for the target to double its size, the creature or object attains the maximum possible size in the space available.

Which would suggest a Medium creature does not become Large/larger/taller/heavier...
...unless they do and they are now squeezing in the space and/or overlapping with the other items and creatures (provided they were not already squeezing)?

The Enlarge effect goes on to state:

The target's weapons also grow to match its new size. While these weapons are enlarged, the target's attacks with them deal 1d4 extra damage.

Which would seem to mean one of a few possible outcomes:

1. Creature size isn't changed, weapon size isn't either, no +1d4 extra damage, only bonus to Strength checks and saving throws
2. Size is changed, creature is now squeezing and must make or find a spacious enough area where appropriate
3. Undefined; GM decides

Answer 1

If possible, the creature grows and is squeezing. When it grows, its weapons grow accordingly.

Let's say a Medium creature is in a 5ft-wide tunnel, and is Enlarged. Then it is now Large and squeezing:

A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide.

On its turn, it can move away if it has a chance, and stop squeezing.

If the creature can't squeeze, then the enlarge/reduce spell won't make them grow. For example, let's say the creature was a Large polymorphed druid, already squeezing for some reason, and in the same 5ft-wide tunnel. When you Enlarge, nothing happens. On its turn (if Enlarge is still in effect), if it moves to a space where it can grow, then it grows to Huge, and its weapons grow then (as only now did it attain a "new size"):

The target's weapons also grow to match its new size.

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When you mention your target being surrounded by enemies, a creature can't usually overlap with other creatures' spaces.

Whether a creature is a friend or an enemy, you can't willingly end your move in its space.

One can argue that a creature isn't finishing its movement there, since Enlarge is an external effect from another creature. So by RAW, the creatures could overlap. However, as a DM, if I'm playing on a grid, I'll avoid having overlapping creatures (with the exception of Halflings) just to keep our minis and tokens from jumbling all up together.

Answer 2

The creature expands into the space of the other creatures

A medium creature surrounded by 8 small/medium creatures will occupy its original space and the spaces of 3 of the surrounding creatures in one of the corners.

The rules state:

Whether a creature is a friend or an enemy, you can’t willingly end your move in its space.

There is no prohibition on creatures sharing a space, only on them voluntarily ending their move in that space.