How much are you feeding? Higher calorie foods simply need to be fed less of, sometimes certain dogs are more prone to gaining weight than others. I would first try to just reduce the amount you're feeding. For reference, my super active 15lb Yorkie eats 1/2 cup per day of Acana (and Orijen) kibble mixed with a bit of wet food and is at a perfect weight for his frame.
But I've heard people have had good results with Wellness CORE Reduced Fat for weight loss. Natural Balance tends to be lower calorie and has a lot of options but you're also sacrificing a lot of meat. If you wanted to stick with TOTW, they have a new lamb flavor out now that is lower-calorie and lower protein (I think around 25%). Just some ideas.
