Denim and duck cotton tend to "break in" after a few wearings. This happens with a lot of cotton work clothes and also with a good pair of technical or work boots. They tend to mold themselves to your body shape. It doesn't happen at all with nylon work pants like the ones Carhartt makes.

All jeans relax by about 1" in the waist after a few wears, because you're literally just mechanically stretching them every time you sit down.

Cotton tees would stretch like that too if you wore them as tight as jeans are worn around the waist. But we don't wear them that way.

That's probably not the washes but due to wear.

If somebody wants to wear tight jeans, how can they do that, if the jeans that were tight in store expand afterwards? Surely you can't buy a size smaller because then you can't wear them to break them.

70s and 80s movies come to mind where girls would be getting ready and lie down on the bed and suck in to get the button buttoned. Dazed and Confused comes to mind where they literally use a pair of pliers to put up the zipper because its so tight. Until you break it in, you are just having to suffer. Once they put elastic into jeans, people jumped on it because its just easier. I love my old-style selvedge denim, but it is more work and care than throwaway stuff.

> how can they do that

The answer is literally, with great difficulty. Anything tight that needs to be broken in is reaaally tight at first. But when there's a will, there's a way.

If the jeans have elastane (which most tight cuts have), it should help.

As an aside, skinny jeans haven’t been fashionable in years.