And now for a solution to let my jeans out? Way out.
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Turn them wrong side out. See where one side is just a plain seam, sew another seam a half inch inside that one. Do both sides. Turn them right side out again. Try them on. Better? If not do it again. You may want to take in more the closer to the cuff you get. Depending on the jeans and your build it may be better to take in some on both sides of each leg. When you get them where you want them, sew another seam just outside the one you like for extra strength. If the waist is too big then you can try the same thing but it's much harder to make them look right.
Are you trying to make the fabric more plyable, or are you trying to put holes in the knees like a pair of 80's style jeans? I know in hollywood when they have to thin a piece of fabric to place a squb for a bullet wound, they thin the fabric by combing it with a razor blade. It works for small areas where a small explosion will tear through the fabric and release fake blood. It keeps the appearance of normal fabric, but its thinner than the rest of the materal.