wingsandsword
Legend
This seems like a strange ruling in light of the fact that “a door” is literally one of the listed examples of objects.
A door, yes. . .in the sense of a normal door for a dungeon room or a house.
A gigantic fort gate, no.
Maybe the rule from 3.5e on the equivalent Shrink Item spell would be a guideline. In 3.x, the equivalent spell reduced an item that could be up to 2 square feet per caster level. You could shrink a door to a cabin with that. . .but probably not the gates of a fortress.
While people like how 5e is fast and loose with rules sometimes because it can mean faster play at times, the more codified, fixed rules of 3.x were written specifically to anticipate this kind of question.