The only way that I can see relation to arm size is if you have too narrow of shoulder width on your jacket you can see your shoulders and biceps protruding out from the arms. It makes you look like you have two shoulders (one where the shoulder padding ends, and one where your actual shoulder curves). Logically speaking, my idea is that shoulder seams on your shirts can be farther apart because since it has no padding, the seam will rest on your back and have more distance to cover. A suit's shoulder line will be a more straight line (unless it is completely unpadded) and thus can be a little more narrow in measurement. I'm only thinking this because of geometrics... the distance from two horizontally adjacent corners on a square is considerably shorter than the distance between two diagonally opposite corners. Correct me if I'm misunderstanding your question. An example for me is that my shirt shoulder width is 18" or so, while my suit shoulder width is on average 16.5". By my logic, this is explained by the fact that I prefer a very square, structured shoulder on my suits. The same goes for tighter fitting jeans that taper down to the knee and then back out for a straight or boot cut. A completely straight inseam will appear a longer because its going straight from the crotch to the ground, as opposed to an inseam that contours to your leg. For this reason, I wear 32" inseam on straight legs and 33" on slimmer or bootcut jeans.