The reason this occurs is because the id on the product being added is the same, so the cart interprets them as the same item being added. You will have to bind the id dynamically to a value set by the select box for the different sizes to be added to the cart separately. So if you go into your add to cart button server behavior and bind the id to the select box's value (as long as the value is an integer), then you will have multiple items added to the cart.