Pet peeve (or worse - I DETEST) when a pair of pants fit perfectly at point of purchase, but 'grow' over the course of the day! They bag, sag, and I find myself constantly hitching them up at the waist. I can only wear them once, and then have to wash them in order to shrink them to fit.

Jeans in:
100% cotton
cotton/spandex
cotton/spandex/polyester
cotton/spandex/viscose/elastane (1%)

Stretchy pants in:
Viscose/nylon/elastane4%

...have all been GOOD though the knees in the stretchy pants bag a little by day 3-4. I have passed on pants and jeans that 'grew' and now don't recall the fabric mix. Viscose and polyester don't seem to be the culprit. I'm wondering is elastane (in high percentages) or modal is the problem.

Any ideas what causes a garment/fabric to 'grow' with body heat? And what I should avoid in order to circumvent this happening?