My youngest is also very thin. She was starved as a baby, but gained weight in leaps and bounds after we rescued her. I thought she would never stop growing. But, she did - and she was still very thin. Better than emaciated, but thin. Now, we have had her over a year and she is one year and four months old, but only weighs about 48 pounds - but, she's tall so she looks so skinny! I feel awful about it. She eats exactly the same amount as your doggie - 2 1/2 cups in the morning and the same amount in the evening for a total of 5 cups a day. She won't eat any more than that. No table scraps, though - makes her belly hurt. When we got her spayed, she lost what little extra she had gained and now is back to super skinny. Like you, the vet says she's fine and too thin is way better than too heavy. So, don't feel bad - it's not just you! Here's hoping our babies will grow up to be big healthy-looking boxers!