It may be both, and a short test programme will tell you which. Bagging at the knees is usually about fabric growth, the stretch that does not recover. Bagging at the waist can also involve the waistband elastic. Heat during drying is one factor that can reduce recovery, but construction, elastane content and finishing also matter. Test before you decide.

Measure growth and recovery, before and after

For the body fabric, ASTM D2594 covers stretch properties of knitted fabrics having low power, including fabric growth. For the waistband elastic, ASTM D4964 measures tension and elongation of elastic fabrics on a constant-rate-of-extension tester. Run both on new material, then on material that has been through a named laundering and drying procedure. ISO 6330 includes defined drying procedures, among them tumble drying. Compare new with dried results.

Read the result both ways

If recovery stays acceptable after the drying your label allows, the complaints may reflect hotter drying at home than the label states. In that case clearer care advice is a reasonable response. If recovery drops noticeably even under the labelled drying, the label is promising too much for this fabric. Either change the care instruction to a gentler method the fabric does tolerate, or ask the mill about a construction or elastane that holds up.

Keep the label within the evidence

ISO 3758 symbols indicate the most severe treatment that does not cause irreversible damage, and US rules expect care instructions to have a reasonable basis. A "tumble dry low" symbol or instruction should therefore be one your tests support. If testing shows that only line drying keeps the leggings in shape, the label should say so.

Decide with the product in mind

Activewear customers often machine-dry everything. A fabric that tolerates low tumble drying may be worth more to your returns rate than a slightly cheaper one that needs line drying. Weigh the test results against how your customers actually care for the product.

Whatever you decide, record the tested drying condition, the growth and recovery results and the approved fabric reference together. When the next lot arrives, you can check it against the same numbers instead of waiting for reviews.