Hats becoming very large after wearing just a few times

I am a new Knitter. I made two hats last year and both have become way to large for the wearers. Do you know what I may have done wrong

What are they made of?

One possibility is the yarn you used. If it was acrylic. stretching should be expected. Another is the stitch you used in the “hat band” part of the hat. With wool, “k2 p2” (on round needles) usually gives a nice tight band that should resist stretching.



As Landolphe says…

If the hats are acrylic, you (or your friends) could try running them through a wash-and-dry cycle with hot water. Might help, might not. Or try the elastic trick. Thread a tapestry needle with elastic thread and make 3 or 4 rows of running stitches inside the edge of the hat, making sure the elastic doesn’t show on the outside. Pull up the elastic until the hat fits snugly and tie it tightly. (Essentially, you’re gathering the edge of the hat to make it tighter.)

how do you do a running stitch? I know how to do a duplicate stitch could I use that to gather an old sweaters ribbing? Would that pull it in tighter?

One hat was wool, the other acrylic, they were both made using a rib stitch.
what is a running stitch?