The wood contracted.
See the second definition.
1. To enter into or make an agreement: contract for garbage collection.
2. To become reduced in size by or as if by being drawn together: The pupils of the patient's eyes contracted.
=== edit Added antonyms and usage ===
For other opposites, try http://www.synonym.com/antonyms/expanded/.
Antonym of contract (Sense 9) =>abridge, foreshorten, abbreviate, shorten, cut, contract, reduce"
Here, I'm using expanded and its antonym as a transitive verb.
He expanded/abridged his novel.
She expanded/foreshortened her perspective.
He expanded/abbreviated his speech.
She expanded/shortened her trip.
They expanded/cut her research budget.
Here, I'm using expanded and its antonym as an intransitive verb
The bridge expanded/contracted as the temperature changed.
His waistline expanded/reduced with exercise.