I've got both of my cats exclusively on canned foods (switch between Wellness grain free and Performatrin, Pet Valu's brand). Jake was large, and has been losing weight steadily since he's been on the canned food (I feed him about 200 calories a day - 4/5ths of a can of Wellness) and I have a growing kitten. We give them about the same amount of food (Alley, the kitten, is a lot smaller at 4.5lbs, but since she's a growing kitten, she gets the same amount).
As I said, Jake is losing weight, and Alley is doing fine. I wouldn't worry about them both losing/gaining, if you stick to the right amount of calories for their ideal weight (ideal being the appropriate word).
Jake is getting weighed tomorrow (couldn't find a baby scale for the life of me, so we're bringing him to the vet), and if he has reached his ideal weight, or is a little less, we'll up the calorie intake a bit, but right now both are doing great!
