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I've noticed that around a couple hours after I tuck in my dress shirt to my suit, the sides and back will begin to puff out. And even when I tuck in the sides, the back will have a lot of excess shirt.
Notice that even if the shirt was completely tucked into the sides, the back would still be very puffy as a result of the excess shirt material. I am around average height and weight (5' 11", 160lbs) and I have already tried buying slim shirts (which mitigated, but did not solve, the problem).