Have you ever wondered why some apparel looks great on some while not on others? There may be many answers to this question. For the time being, let's look at below article for one the contributing factor - neckline:

The neckline can frame the shoulders, neck and face, and change their apparent width, height and angularity. It is generally advised that the neckline shape be chosen to balance the wearer's natural features.

Thus, a face with round curves is well-framed by an angular (linear) neckline and collar, whereas an angular face is softened with curves. Similarly, a short neck and face will appear lengthened by necklines with strong vertical (or nearly vertical) lines, such as a plunging V neckline (or a long knotted scarf or necklace); conversely, high necklines such as the jewel or turtleneck types balance long faces.

Broad shoulders can seem more narrow with style lines that converge inwards (e.g., the halter neckline), whereas pear-shaped figures can be balanced by diverging or horizontal lines, e.g., an off-the-shoulder or boat neckline.

The designer should also consider the amount of décolletage the wearer would like and whether the visibility of undergarments (such as bra straps) is relevant. These factors may influence the depth and width of the neckline, respectively.