Because of their position and form the abdominals bulge in relaxed state and the abdominal wall is pulled flat when it contracts. If you improperly train your abs by neglecting other supporting muscles your abs will protrude.
The Pelvic Girdle is a group of internal muscles that support your internal organs. During exercise your internal organs can shift and change your center of gravity. You can easily experience this when doing hanging leg raise your body starts to swing. The proper way to do abs workout is when you engage your pelvic girdle, strengthen your center of balance (no swinging or momentum) and force more tension on your abdominal wall. The pelvic girdle is tightened by clenching your insides - squeezing your rectum or performing the same action as cutting off the flow of urine. This exercise is known as Kegel. By performing Kegel you ensure not only that your pelvic girdle is strengthening but you’re also building strong core and center of balance.The Transversus is the other abdominal muscle that serves to pull your abdominal wall back. The transversus is contracted when you suck in your gut or try to pull your belly toward your spinal cord. If you workout without contracting the transversus, the abdominal wall will increase in mass and instead to form nice six-pack it will bulge and protrude.The Obliques are the muscles that run diagonally across the abdominal wall and are attached to bone your sides. By increasing the size of your obliques you can increase the width of your waist. These are also called "love handles" some people avoid training them for these reasons. But even if training obliques can increase your waist size, having strong obliques means that your abdominal wall will be pulled flat.
And finally - what is the proper way to do abs workout?
The abs are not special, they not require constant training. These muscles form your core strength and almost any activity will work these muscles (bench press, running, squats).
Training your core once or twice a week is enough for most people. Because of the role of these muscles they are building mostly of "endurance" fiber and respond better to higher reps workout (12-15). If you are doing 100 crunches.....well that means that you’re using other muscles too (hip flexors). Make sure you perform vacuum and Kegel prior to begin any new set of exercise. The proper form here and concentration on above described techniques are more important than heavy fancy exercises. You can build a nice six-pack using only body weight exercises.