If you want to rule whipworms you may want to get that test done. Otherwise, I think it is just like people - I got lucky and got the "thin" gene from my mother's side, my dad and his side of the family have to really watch their weight - it is easy for them to gain weight. So your dogs though related have different metabolisms as they way I see it. As long as your thin dog is healthy - I won't worry about the weight.
Sting Chance von Gaard AKC GSD 2/8/2006