My healthy German Shepherd eats a lot but stays thin. Is this normal?

Yes, some dogs are just thin! It is good you noted her stool is normal and that she is eating well. Before I chalk it up to being naturally thin, there are a few things I would want to make sure of including parasites, disease and nutrition. It is always a good idea to rule out parasites with a stool sample to the lab since they can be microscopic. GSD are also prone to a disease that can make it very hard for them to gain weight but typically these dogs have 'greasy' looking stools. There are tests to rule this out. I would also make sure she is on a high protein food- your vet may recommend a prescription working dog food if she is thin to the point of concern. But if all this checks out and she is naturally thin- then I would be glad to have a dog that watches her weight:) I hope this helps!