This is a common pattern in cotton jersey. During knitting and finishing, the fabric is pulled lengthwise, which stretches the loops. In washing and tumble drying, the loops relax back toward a more natural shape: they get shorter in length and spread a little in width. The result is a tee that loses length and gains width. A single shrinkage figure such as "within 5%" hides this, because it does not say in which direction.

What controls it

Mills control dimensional change through yarn and knit choices and through finishing, especially relaxing the fabric and compacting it, which pre-shortens the fabric so less shrinkage is left for the customer. Heavier, tighter knits and well-compacted fabrics are usually more stable. Fabric stretched to gain width or length on the stenter, for example to improve yield, tends to move more after washing.

Set limits for each direction

Write separate tolerances for length and width, for example a maximum shrinkage in length and a maximum change in width, both measured after a named wash procedure such as ISO 6330 at your label settings and a stated number of cycles. Include growth as well as shrinkage. Ask for these on fabric and on finished garments, since sewing and garment washing can change the result.

Allow for it in the pattern

If some shrinkage is expected, the pattern can allow for it. Agree with the factory whether measurements in your specification are before or after washing. Fit and approve washed samples so the shape customers get after washing is the one you approved.

Check bulk

Check fabric weight against the approved fabric with ASTM D3776, since a lighter, more stretched fabric often moves more. Wash garments from bulk in each size and measure body length, chest width and sleeve length. Bow and skew can be checked with ASTM D3882, since twist and dimensional change often appear together.

If bulk moves more than the limits, ask the mill about relaxing and compaction on the next lot, not just a pattern change. Keep the washed measurements from each order, so you can see whether a new fabric lot behaves differently from the one you approved.