In evolutionary and biological terms, a small waist signifies youth and vitality. But doing endless workouts of crunches alone will not allow you to achieve that Venus-like waist of your dreams. You will need to supplement specific waist-orientated exercises with ones from various disciplines.
Specifically you will need to work the inner girdle of muscles - commonly known as the core muscles - with slow abdominal exercises that work very deep into these muscle groups.
While usually associated with building flexibility, some specific exercises from yoga-based workouts also target the underlying core muscles around the waist.