Working your abdominal with heavy resistance, just like we do any exercise, will increase their size, again just like any other exercise we perform. When we do this, our abdominal does not get 'thick', it actually grows each individual segment of the 8 segments we have (Yes, when you do ab workouts, you work all 8 segments, and not the 'upper' or 'lower' abs, thats all just new age mumbo gumbo) and thus makes your abs bigger, and your sixpack is more bigger. All professional bodybuilders do abdominal workouts with heavy weights, and look at their abs, they are to die for.