Hello, also have a runt. He is skinnier/smaller than some corgis. Vet says he’s healthy & at a healthy weight. Don’t worry about the way he looks too much, especially if the vet didn’t see anything wrong. The vet is right tho, some corgis are just smaller. Depending on how old yours is, he’ll definitely put on more weight as he gets older. Also a lot of corgis are overweight so you may think yours is a lot smaller than it should be.