The design and purpose of this shirt are the reason why they must have a moderate fit on any body size. It must neither be too tight to show off your muscles nor too loose and baggy. There are times when you might need to add a layer of clothing below or above these shirts. So, make sure the size allows enough space to do so. Meanwhile, if you are looking to wear a sports t-shirt, you must consider the movement of your muscles. Specifically, if you are using them for athletic activities. At such times you will need to allow freedom to your muscles. So, pick a fit accordingly.