Anyone Ever Had A Dog That Just Seemed To Stay Thin, Even If They Receive More Than Adequate Nutrition?

I have a 7 month old hound mixed breed who is extremely active. She runs constantly when she’s outside, playing with our larger dog (a hound/Ridgeback mix), and she hates coming inside unless it’s to eat. After that, she’s ready to go back out and play until it’s time to come in for the afternoon.
The problem is, Daisy (the little dog), always looks underweight. She has long, skinny legs, and she has very little body fat. You can see her ribs a bit when she’s standing still, and even more so when she starts eating. I feed her approx 3.5 cups of dry puppy food per day, but she never seems to put on any weight. I’m a Humane Society volunteer, so my neighbors are used to seeing me walking the neighborhood w/ underweight foster dogs, but I don’t want anyone to think I’m mistreating my own dog by underfeeding her.
Should I give her more food to “fatten” her up, or could she just be a naturally “slender” dog and more food would make her unhealthy?http://i8.tinypic.com/625cb38.jpg

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