Laundry Reviewed says that high temperatures cause denim to shrink. Washing jeans in hot water or putting them in the dryer on high temperature, will make them tighter.
Be cautious when attempting to shrink jeans, as making denim tighter often damages the material. Consider keeping the denim safe by washing it in vinegar and letting it dry outside. Raw denim naturally shrinks to fit the body after the garment has been worn and washed a few times, high temperatures or not. Consider the long-term life of the jeans before throwing them into hot water or into the dryer for an hour or two.