I agree that looking at your photos, she doesn't look too thin. She looks in similar body condition and probably size as our Baxter. Mind you, Baxter is only 4 months and weighs 35 pounds, but he is apparently tall for his age. Sometimes when I look at him I think he is a bit thin but the vet says he is in perfect condition for a growing pup. From what I can gather, Boxers can grow at very different rates and perhaps your girl is just a slow developer. However, I would probably just get her checked out to be on the safe side and give you peace of mind. Good Luck!