Form Fit T-Shirt?
Form fit is more than mere size, it's how apparel conforms to your body and how the person wearing it interacts with it. Is how fabric conforms to your body the sole determination of its fit? I say no.
One of the things for me to learn was when to wear which t-shirt. Before I got into tie-dye, a shirt was just shirt, until I discovered the weight of fabric in shirts. Ever notice how a shirt falls from your shoulder and the rest of your torso and arms? The weight of the fabric determines how it will hang on your body. For example, lightweight shirts have more of a draping effect, whereas mid-weight shirts have a more structured effect. Selection of the right size for how you like a shirt to feel determines how the shirt conforms to the body. The right weight of a t-shirt opens more occasions to wear a casual t-shirt.