Is a container full of objects a valid target for the Enlarge/Reduce spell?

Question

In the last session, my party and I had to rob the major of a city. We needed to retrieve a particular object that proves his actual corruption by the local thieves' guild.

We managed to get into the major's apartment at night, in "stealth mode", and we entered in his office: we came to know (by Locate Object) that the object was in a locked chest, so large that the DM said "you need two people to carry it, and your speed is halved since it is really heavy".

Unfortunately, while we were trying to open the chest, something went south and we had been discovered by the guards: we had anyway a plan for a quick escape (broke window plus Feather Fall). I remembered that I had Enlarge/Reduce prepared, then I cast it on the chest, thinking to reduce it in both dimensions and weight, for carrying it with us during the escape.

The DM said that even if the chest is an object, it was full of other objects, and the latter are not affected by the spell: indeed, the text of Enlarge/Reduce says:

If the target is a creature, everything it is wearing and carrying changes size with it. Any item dropped by an affected creature returns to normal size at once.

and it does not say anything about objects in a container. Hence the DM ruled that the chest did reduce, since it is a valid target for the spell, but the items inside it remained at their original size. This resulted in the explosion of the chest, because the "pressure" of the inner objects on the shrinking chest made the container explode. In this way, all the objects were on the floor, freed, and we grab what we needed.

At the table, we agreed that it was a reasonable ruling, which was quite fun as described by the DM and moreover allowed us to achieve our goal. Setting aside for a moment that a DM can interpret the rules as better suits the table, we (the party and the DM) still wonder if this was in agreement with the description of the spell, i.e. if a full container is a valid target for Enlarge/Reduce.

Did we miss something in reading the spell that clarify the case at hand?

Answer 1

Rules as written, the spell only affects a single object

Technically, yes, the container full of objects still is an object, and with that a valid target for Enlarge/Reduce.

The question is not if you can target it, the question is if the items within are also affected by the spell or not, when the spell normally can target only a single discrete object. Enlarge/Reduce says:

You cause a creature or an object you can see within range to grow larger or smaller for the duration. (...) If the target is a creature, everything it is wearing and carrying changes size with it.

The spell does not have a clause that extends the effect to enclosed objects like the one about worn or carried objects on a creature, so it will not shrink them.

Object Definition

The question remains if maybe an object includes enclosed objects, so that there would be no need to extend the stipulation in the spell? You have the same question when you disintegrate a chest full of treasure, or when you teleport an object. The consensus is that enclosed objects are not part of the object.

Supporting this (thanks to @Play_Patrice) is the observation that objects in the container would have total cover from the spellcaster, so there would not be a line of effect to change them.

Unfortunately, there is some room for doubt (also showing in the secondary answers). The object definition says it must be "a discrete, inanimate item" (singular) that may not be "a building or a vehicle that is composed of many other objects", but the rules are inconsistent on this (DMG, p. 246/247): example objects include a lock and a cart, both of which consist of many other objects. The lock consists of tumblers and housing, the cart consists of axle, wheels, frame and even is a vehicle. So the DM must make a call, what is, and is not, counted as an object for these spells.