There are two kinds of stomachs, one is normal, just like a person's stomach is too big and thick than an adult's because of the large size of the liver in infancy, and a puppy will have a round "big" belly; The other is pathological enlargement, generally due to serious parasites in the body, which leads to hepatosplenomegaly and even ascites.

As for thin legs, some breeds of dogs have thin legs, such as baby deer.

So you should first compare your dog with the dog of the same breed. Judge whether it is indeed an abnormal situation.

If it is caused by serious parasites, the stomach is very big, malnutrition, and then "thin legs" and "unstable walking" appear, please see a doctor immediately!