It has to do with size too.. the bigger they are the lot less leaner you'll have to be to display them... mind you the obliques are a small muscle group and you don't wanna widen your waist. I suggest you use actual weights for any abdominal exercises as opposed to just bodyweight crunches or things of that sort. It is still a muscle and responds best to weights in maybe the 20-30 range. If your doing sit-up for 100+ reps that just seems pointless to me... you should feel them burning around reps 20 or so otherwise your just wasting your time doing pointless reps with little weight.