I have seven, all are beautiful examples of the breed but they are all shaped just a little different. The three females, (daughter, mother & grandmother) are all built different. All eat the same amount of food (about two and a half cups, once a day) and all exercise the same amount. The four boys are the same way (dad, two sibling pups and Bleu) all different shapes and sizes. And yes, one of them has always been skinny (D.O.G.) he gets more like 4 cups a day and just wont put on any weight. He still looks good, he's just a bit thinner, his brother (Punkin) was sold as a pup then due to a crazy work schedule, he had to re-home him so I took him back. He was a little overweight and it has been hard getting the weight off of him. If the vet has checked them and said all is good, I wouldn't worry about it.
Haven't had it in years but never spit any out.