fitted-cut t-shirt has a curved side seam which comes in at the waist and goes out at the upper and lower ends, to make room for curvy hips and bust/chest areas. The shoulders are also cut narrower, and the sleeves are shorter and more fitted relative to a straight-cut shirt.
Regardless your body shape, the perfect T-Shirt should neither be too loose nor too tight. A T-Shirt that is too wide does not hide unnecessary kilos, but makes the wearer look even bigger and round.