It's actually very difficult to add muscle mass while running a calorie deficit to lose weight, as the body burns protein for energy, rather than creating new muscle tissue. So it is extremely unlikely that ab exercises will make your stomach look bigger.
But it's equally true that ab exercises aren't going to make your stomach look smaller either. Energy is delivered to the muscles via the bloodstream, NOT from being absorbed from surrounding fat stores. Ab exercises will not burn stomach fat.
So when should you be doing ab exercises?
Well, without strength training, up to 25% of weight loss can come from lost muscle rather than fat. And its a lot easier to maintain your existing muscle mass through a regular program of strength training, than it is to add it back later.
But ab exercises (eg. crunches, situps) only work a very small proportion of the core muscles - you would be much better off doing something like planks, which work a much broader range of core muscles. And a strong core is important for posture and balance, and makes cardio easier.
Bottom line: you should be strength training your core NOW!
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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