Sleeve widths and lengths are in proportion to the size of the shirt -which corresponds to the measurements of the body the size is designed to fit. So the sleeve circumference of a small is smaller than the medium and the large will be larger than the medium -and larger on upward through the size range. The assumption is that if the girth around the chest and waist is larger then the girth of the bicep of the arm is proportionally larger, too. For 85% of the population this formula works and that enough for the clothing industry to keep the measurement rule as the standard.
no, because if you need an XL t-shirt you are generally bigger all over than someone who needs a S tshirt.
