Each of my products come in different sizes (I have an add on per size)....each size weighs a differnent amount.
How can I make the shipping calculate based on the weight of the actual product add-on?

So for example product

0001 weighs 1 lb
size small = .66lb
size medium = 1.5
size large=2

When it calculates the weight for shipping it is using the 0001 weight, instead of the weight of the actual size (add on) that was selected.

Thanks!