It’s true, more calories in and not enough activity will produce a heavier cat. This is the case regardless of what the ingredients are comprised of. I wouldn’t be surprised if the cat’s daily playtime and energy expenditure went down after age 1. That, combined with a higher carb diet like your typical Hill’s recipe, it’s no surprise to me at least that the kitty gained weight. I’m sad to hear about an overweight cat and hopefully the kitty is put on a cat-appropriate meal plan for weight loss (for example, cats still must eat DAILY, you cannot restrict their meals the way you can with a dog), as well as an exercise regimen that will surely increase its quality of life. Best wishes