It wasn't the fat content that made your dogs fat. I don't even know the ingredients on the product you're talking about but I can tell you that dogs need fat for energy (esp boxers). They use fat like we use carbs. The food probably has several grain type ingredients which makes it too high in carbs. Again, I don't know the formula but the "light" version will probably make them even fatter. Most "low cal" or "low fat" formulas also mean low meat content & they put even more carbs into it. Also too low fat (less than 10% isn't sufficient to produce a healthy coat, so in addition to weight gain, you might notice a dry, flaky coat. Trader Joe is not a pet food manufacturer. They contract with someone else to produce a product with their name on it. Knowing the problems with the pet food industry nowadays, esp after the 2007 recall, my guess is that their food is made by somelow quality 3rd party. Costco's Kirkland brand is made by Diamond, the poster company for pet food recalls & dirty tricks. The ingredients may look good & they may even be good but too many grain ingredients are not good for dogs...they need MEAT, not cereal.