Cotton shrinks when washed or dried. Cotton fibers and polymers are stretched when made into thread and cloth, and washing and drying shrinks cotton back to its natural size.
The best method to avoid cotton shrinkage is washing the garment in cold water and air drying. One-hundred percent cotton is the most prone to shrinkage. Cotton and polyester blends shrink less. Cotton can be pre-shrunk, which means it was washed prior to construction of the garment. Another type of cotton is shrink-proof, which is treated with shrink-resistant or durable press finishes that prevent the cotton from relaxing when it's washed.