The idea that leg lifts, situps, and crunches will reduce your waistline IS A MYTH. These exercises mainly activate a small, superficial muscle layer ('rectus abdominis') at the front of your torso. Focussing your waist-work on this muscle can affect your posture (making your belly bulge more) and eventually damage your lumbar spine.
The muscles that define your waist are used to turn your ribs and hips in opposite directions. They CANNOT be properly trained while you lie on your back.
To tighten these muscles, you need to perform activities that involve a lot of WAIST-TWISTING, and HIP-WIGGLING, while your torso is vertical or constantly changing position.
Try salsa dancing, kayaking, high-kicking martial arts, swimming 'freestyle' (crawl), exotic skipping ('jumping rope') moves, and walking like Mae West. Capoeira uses a lot of twisting and bending movements, and is an excellent waist-work activity.
You can buy a very inexpensive 'twister' turntable, to use at home. This is a small version of device that some gyms have, with a handle at the front so that you can keep your upper body still while rotating your lower body from side-to-side. This under-rated machine is an excellent fitness aid.
A pilates class can also help you to change your postural habits, by teaching you to perform a movement called the 'pelvic tilt' or 'pelvic curl'. This will allow your 'core' muscles to co-operate better, and re-focus your other waist-work so that it 'pinches' your waist in.
Your weight implies that you might not be eating enough to maintain good muscle tone. Severe dieting causes muscle 'wasting', as a way of economising on energy output. Core muscles that have been weakened in this way can be unable to prevent the internal organs from bulging outwards.
Increasing your energy intake might allow you to add muscle mass to your upper and lower body, as well as helping your core muscles to compress your abdominal organs properly. This strategy will make your waist smaller, as well as making it look even smaller because of the athletic curves above and below it.