Sometimes, on my day off or on the weekend, I’ll wear the same pair of pants 2 days in a row. Of course, this depends on if I got them dirty or not. Anyway, I’ve noticed that towards the end of the second day, the pants fit looser, like they stretched. I would just assume they’ve stretched a bit from being worn extra long but…after my wife washes them, they’re back to their regular fit. I’ve noticed this my entire life, regardless if the pants were jeans, shorts, etc. What’s going on here?
You just answered your own question. They stretched. When you wash them, they shrink again.
But why do they stretch? Because you’re wearing them. Duh!
Wearing clothes is like using a really slow steam iron on them. You constantly sweat (even when you don’t get “sweaty”, except women- they just glow a little) and your body heat loosens up the fibers of the fabric. You stand up, and sit down, and just generally move around: this puts stress on different parts of the garment. It puts a little stretch here, and a little pull there. After long enough (by the second day) your clothes are all stretched, and fit funny. Or better, depending.
If you go to a good tailor (or make your own clothes) you get your pants “pre-stretched”. They steam the fabric for the seat of your pants and stretch it a bit before they cut and sew it. It keeps your pants looking nice all day.
This is just the long form of msmith537’s answer.
-Rue.
And it is my sad duty to inform you that as you age, the opposite phenomenon occurs. Clothes shrink. Because of course, it’s not us growing!
Yeesh. I change my pants only when I’m doing a load of laundry, which is like once a month. I guess my pants were originally 5 sizes smaller then.