I don't know about why a potato weight less, but if you're doing the weight watchers thing, I wouldn't worry overly much about it. Just use your common sense. If it's a medium sized baked potato it will be 2 1/2 points, if it's large 4 points. The weights they give you are more of a guide when it comes to something like this. I think the idea of it is to get you into good eating habits. Anyway, this is how I've been doing it, and I've lost 2 1/2 stone. Good Luck, I hope it works for you.