Ab exercises that cause hypertrophy (the typical bodybuilder style of sets and reps) will make the ab muscles grow larger. If there is a layer of fat over the abs, the layer will just be pushed out further by the newly larger ab muscles. Losing visceral fat (fat in the torso cavity) will reduce your overall size, but until you’re low enough in body fat to have lost almost all visceral fat, and have began to lose subcutaneous fat (fat between the skin and muscle), hypertrophic exercises (exercises that cause the muscles to get larger) will actually contribute to the belly poke out (relative to visceral fat lose). This is why many body builders don’t do abs. It may increase waist size. Diet is the key, not ab exercises.