Here's the issue with "stones." We learned the hard way with our males - they all had crystals in their urine over a several month period and blocked.
Male cats have very narrow urethras, and if they develop crystals beyond normal levels in the urine, it irritates the bladder, and they can get blocked with sloughing of cells from the bladder wall, mucous, and the crystals.
It is a genetic proclivity and the pH of the urine - and the concentration of the urine that determine whether or not a cat will have a problem with crystals.
For a while there, the most prevalent type of crystal in cats' urine was struvite crystals. These are created in urine that is not acidic enough. Whenever you see "bladder health" on a non-prescription cat food, that means it has acidifiers in it. BUT... when all the cat food manufacturers adjusted their formulas, then cats started having problems with calcium oxalate crystals. These are created I BELIEVE, if I remember correctly, in a bladder that is low pH combined with higher magnesium levels.
The bottom line is that a kibble-based diet usually leaves cats with concentrated urine. The number one thing that can be done to reduce the risk of crystals in urine is dilute the urine. And the vet prescription foods for urine health NOW target a pH neutral urine.
But they are typically not very healthy foods, and are loaded with carbs, and yes, they are a cause of weight gain, and thus a risk for diabetes.
If you think about what a cat eats when not owned by a person, in the wild, they hunt small rodents, rabbits, birds - and depending upon where they are, they'll also eat things like lizards. They do not graze on grass, they do not raid the garden, they do not eat corn. They may derive SOME minor nutrition from the contents of the stomach of an animal, but as Stefan pointed out, this is a very, very, very small amount of grass or grain or whatever is in there.
Cats are carnivores. Their digestive system is so finely tuned to eating meat/bones/organs of other animals that they are unable to derive nutritional value from things WE think of as nutritious. Take carrots for example. They do not possess the digestive enzymes necessary to process beta carotene into vitamin A. They MUST get their vitamin A from animal tissue.
That said, the highly processed kibble and canned food we feed them is cooked. This destroys a lot of the nutrients they need from the meat that is in the food. So supplements are added. EVERYTHING ELSE is in there because it's cheaper than meat, and it appeals to US. Cats do not need, nor do they eat things like carrots, peas, potatoes, corn, etc. etc. etc. Their digestive systems are not designed to properly utilize them.
Given the choice between dry food and wet food, the way my vet puts it is that it's better to feed them the cheapest wet food than the most expensive dry food. I'm not so sure I believe this... but I take the point. Some meat and a lot of moisture is better for them.
I decided to improve the quality of the food my cats eat. I transitioned them off kibble altogether - and they were on the prescription kibble for the urine crystal problem they'd had. I was feeding them some canned food twice a day - they weren't big fans. But I got them on timed meals with wet food only. Then I took them off the prescription food, and raised the quality of the canned food they were getting - I purchased the grain-free brands that were mostly meat-based, like Instinct and EVO. I added a tablespoon of water to the wet food, and I kept their urine VERY dilute. They never developed a problem with crystals, even on that non-prescription diet. I have no idea what the calcium or phosphorous or magnesium levels are in those foods.
My cats are older: most of them will be 10 this year. They've been on an all wet food diet for 1.5 years. I found that my gang need FAR LESS food than recommended on the feeding guides on the cans. My guys need only about 4 ounces a day. And one of my cats - at 14 pounds - is FAT on THAT amount of food.
When I transitioned from the free-fed kibble, I was CERTAIN I was not feeding them enough food. I fed them twice as much as they needed, and they all started gaining weight! Yes, that pitiful amount in the bowl three times a day (I feed mine 4 times a day) is enough food! For your overweight cat, you will need to feed more at first - you don't want them to lose weight too quickly. But you should calculate how much food they need for their ideal weight (which is usually less than we THINK it is!!!), and calculate how much food they need for their current weight. Feed for the current weight, then slowly bring it down to the amount for their ideal weight.
If you want to figure out the guaranteed analysis of protein vs fat vs ash and determine the carbs in a food, you need to "remove" the moisture from the calculation to get at proper numbers. Dry food is usually around 10-11%; wet food usually 68% - 78% moisture. When you're looking at the food on a dry matter basis, after you take out protein, fat, ash, and fiber, what you are left with is carbohydrates. So you can use the guaranteed analysis provided by the manufacturer to compare foods, and then plug those numbers into a "Dry Matter Basis" calculator. There is one on this page - in boxes on the right-hand side of the screen, scroll down to "Special Features." The Dry Matter Basis calculator is the second link:
http://catcentric.org/
When looking for a food, you really want to choose one that has the lowest carbs, and the first 3-5 things on the list of ingredients should be meat. For instance, Instinct canned chicken: Ingredients: "Chicken, Turkey, Chicken Broth, Chicken Liver, Ground Flaxseeds, Montmorillonite Clay, Eggs, Peas, Carrots, Lecithin, Vitamins...."
http://www.naturesvariety.com/Instinct/cat/can/chicken
And re: the amount to feed. Our Sheldon was looking thin - certainly leaner than the rest of the cats. He went to the vet for his annual check up, and we were informed that he could stand to LOSE a pound. I think we're just used to looking at overweight cats, and a healthy weight will actually look "skinny" to most of us.
