The prescription diets are a little lower in calories and higher in fiber than the low-calorie over-the-counter diets, so theoretically you can feed slightly more while still cutting calories. But like others have said, the key is not allowing an all-night buffet. My cats are fed various premium-brand dry foods (Wellness, Blue Buffalo, etc.) but I only limit their dry food to 1/2 cup a day to keep them lean. Sometimes they get canned as a treat, but that's in place of their dry food for the day. When the food is gone, it's gone. I've discussed this with many clients with "fluffy" cats and they always seem shocked and I just remind them that the cat is not going to starve to death if there's no food in the bowl for a few hours or overnight. Many of these cats have a full bowl of dry food out at all times and the bottom of the bowl is never seen because it's constantly getting refilled!
Interestingly, there is an internal medicine specialist (for cats) that advocates more canned food diets and no dry for cats that are diabetic and need to get their diabetes under control. The theory is that canned foods, if they are just meat and don't contain rice, wheat gluten, etc., are higher in protein and very low in carbohydrate, which is more the way cats are designed to eat. Cats evolved as strict carnivores and their bodies and metabolism are not really designed to process carbohydrates and starches very well. Might be something worth discussing with your own vet. There are also dry foods out there that are as high in protein, and grain-free, as a dry food can get, and might be helpful - but again, portion control is really important too. Good luck with your "little bit fluffy" puddy-tat! :)