I had the opposite problem recently. Where Levi's jeans expanded and loosened up after a couple of washes. What's the reason for that?
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.