I would say that WEIGHTED ab work CAN make your waist bigger. Especially if you use heavy weights for the obliques. I agree with a columnist in Oxygen magazine, that to build definition and a six-pack (with somewhat protruding muscles at times) you should work the abs with resistance and fewer reps, and to keep a leaner look, go for high reps and little to no resistance. I also think that moves like planks (front, side, rear, with and without added arm/leg movements) and other core stabilization moves help strengthen the abs without adding bulk.
Also consider that different people tend to store any extra fat in different areas. For many women, it's hips and thighs, but some ("apples" like Cathe) tend to store it in the abdominal area. In that case, an overall fat-reducing program (clean diet, cardio to burn fat calories, resistance training to increase the metabolism) will help.