Like others already mentioned, they don’t just differ in size, but in shape as well. What I would prefer is different varieties for each size per sex. For example, I am an Asian medium. So when buying shirts in the US, I’m in that weird spot where a US medium is too wide on the shoulders and the sleeves too big, but the torso is just right. On the other hand, a US small fits just right on the shoulder width and sleeves, but the torso is too tight. It would be great if there are variations based on build as well like pants (i.e. slim, standard, wide, etc.).