Yes, big abs can make your waist bigger but I dont' know about a big back. This is why most people don't add significant weight to their ab routine unless they are strength training. Go for reps and make it lean and stringy. Go for power and make it bulky.
Yes muscle or fat can add size to your waist. Back size will not increase your pants size. In fact, having a huge back gives the illusion of a smaller waist.
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