OMG, listen to us! It's not you, its literally the pants. If you washed them, and they shrunk, that's it. They aren't going to stretch back to their former size and since this isn't a weight issue, the only real solution is to buy a new size up to account for future drying shrinkage, buy a different brand, or just wear something else!
Pro tip: If the fabric of yr leggings looks more shiny after you pull them on, then they are stretching too much. In other words, too small. The fabric should not be any more shiny than it is when it's folded in yr drawer.