You've really got to keep an eye on the AMOUNT of food your cat eats if you want it to lose weight - this is directed at anyone who needs their cat to drop a bit of weight, not just the OP ;}
It's not just about food type, or calories. And you can't just eyeball or guess how much you're feeding your cat - you have to buy a food scale and measure things out - almost exactly. You need to be accurate at least to tenths of an ounce.
You should also weight the cat frequently so you can gauge how its weight loss is going. A pet or baby scale is ideal, but if all you have is a regular bathroom scale, you can weigh yourself, note the weight, then hold your cat in your arms and note the new weight. Subtract your weight from you+cat weight, and voila! Cat's weight :}
A cat should get a certain percentage of its body weight in food per day. The goal is to decrease its food intake until it reaches its ideal body weight. This does require you to figure out your cat's "ideal" weight - your vet can help figure it out, but for most normal-sized cats, it seems like the "ideal" weight is 8-10 lbs.
These are guidelines from the Primal website, but I've seen these numbers corroborated elsewhere:
1.5% Weight Loss
2.5% Maintain Weight
3.0% Slight Weight Gain
So, for example, I have an elderly cat who is pretty sedentary. She weighs 8.5 lbs. My vet said that she is pretty much at her ideal weight, and I should not let her get to 9 lbs (yes, even 1/2 of a pound can be the difference between overweight and just right).
So, I feed my cat 2% of her body weight daily, which is 2.72 ounces. I experimented around with this amount for a couple of weeks, and found out that I could round this up to 3 ounces a day without significant weight gain or fluctuation.
I spread the amount out over 3 meals of one ounce each, and she maintains her ideal weight perfectly. The spread-out meals also ensures that she doesn't gorge on one large meal and then feel hungry 12 hours later, or go hungry through the night.
So, as an example, if you're trying to get a 12-pound cat back to an ideal weight of 10 pounds, you would want to use the "weight loss" percentage of 1.5% of its ideal body weight. This comes to 2.88 ounces of food per day.
As the cat drops weight and reaches its goal, you can then scale the percentage back to a "maintenance" percentage of 2% or 2.5%, depending on its age and activity level.
Of course, exercise is also important. Try to set aside even 10 minutes a day to play with your cat, and consider investing in some toys that the cat can play with on its own, such as the amazing, ever-popular ball-in-a-circular-track-thingy:
Petmate Crazy Circle Cat Toy at PETCO
My cat adores this toy, and will initiate play on her own over and over and over... ;}
Hope this helps!