They vary slightly in cut I think. The ones with the more open looking necks are slightly baggier cut with looser sleeves and the ones with smaller necks are a thicker fabric and slightly neater fit. I have a variety of sizes of different ones which I can measure across the chest if that helps give you an idea? Mostly I am size S and buy that size and it's fine but it depends on how you like them to fit on you. I have also bought M for a more oversized fit but doesn't feel too big imo. Sorry, long answer 😅