purina really isn't fattening.
That's all we ever used to feed our dogs when I was growing up, there were only a handful of foods 50 years ago. We had chihuahuas on purina dog chow that lived to be 19 and never were over weight, we used to feed them once a day (well, more when they were pups). How much are you feeding him? He's probably just a bigger yorkie. Mine has always pretty much been on what people would call "good" foods, and she's big(14 lbs).