Wearing untucked shirts adds volume to your body shape and can distort the proportions of your torso and legs. Business shirts for instance, are long for the express purpose of avoiding becoming untucked. Wearing untucked long tails, especially if you have on an ill-advised tall shirt, make you look like you're wearing an old-timey nightgown.
Also, a common mistake we big guys do, is to tuck she shirt in below the belly. This will only accentuate it! We recommend instead to wear pants with a waist size and rise large enough to pull up to right below your navel. Having the shirt tucked in below your belly will make your belly looked pronounced.
To look your best when tucking in a shirt, use your fingers to pull and fold any excess fabric toward your hips and make sure that fold is held in place by your belt.