Yeah, use added weight on your ab-exercises, keep the rep range in 8-12 for mass like any other muscle you do. Its just dumb to do endless sit-ups withouth added weight, that wont give you those big abs, only small abs which you cant see but with a really low bodyfat. The good thing about big abs(bodybuilder abs) is that you can see them even if you have some fat on them.
Personally i do lots of situps and my abs are defined just fine. Even though my fat covering them preety damn much you can still see them even if i'm not flexing.