not really. A majority of a thin person's waist is gut, so your waist size is dictated by how much and what you ate recently. Of course, if you have large ab muscles or some fat deposits there, it will be larger. In some cases, ab exercises can reduce waist size but only to a certain point at which the muscle cannot become "tighter" and grows increasingly outward. I personally have found no improvement in my waist size through ab training. I eat tons of fiber, so I almost always have a bit of a food baby.