One hundred percent cotton jeans shrink in the dryer, not the washing machine. The heat from the dryer causes cotton fibers to pull together, and jeans become smaller. Cotton articles of clothing shrink only a limited amount, and they do not decrease by two or more sizes.
While people who prefer tight jeans sometimes appreciate the shrinkage, others are able to prevent size loss by hanging jeans up to dry directly out of the washer. Those who use a dryer to dry jeans should set the dryer on a low temperature, remove the jeans from the dryer when still damp, and allow the pants to finish drying while hung up or laid out flat.