Obese cats can predispose to Diabetes, Hepatic Lipidosis and arthritis.

FREE CHOICE FEEDING
THE main reason for feline obesity (as well as obesity in other mammals) is
the consumption of too much food.   Deny it all you want, it is a fact.

What we should do.
Feed two to six small portions daily and control the amounts fed so that
over a period of time the cat does not gain weight.  ..between 0.6 and 1.0
ounce of food per meal for a seven-pound cat. about the same weight as a
mouse.  Cat owners must stop thinking in terms of "cups of food" and start
thinking in terms of ounces of food.

PROTEIN
We all know how cats crave mice and birds as a food source.  A natural
source of nutrition for carnivores, mice and birds are a perfect diet for a
cat.  Did you know that  a mouse or a bird is composed of only 3 to 8
percent carbohydrate and most of that is what happens to be from what the
prey was eating and is in the digestive tract?  The rest is water, a few
minerals, and mostly protein and fat.

What we do...
Many of us purchase dry cat foods, some with food coloring to make it look
like meat and with flour and sugars and preservatives.

What we should do...
We must feed cats a diet with high percentages of protein and fat and low
percentages of carbohydrate (grains) if we expect them to maintain optimum
body weights and a proper state of nutrition.  Protein is THE key nutrient
in a carnivore diet.  On a dry weight basis... where the percent of
ingredients is determined without any water in the ration... a feline's diet
should contain 35 to 45 percent protein, 40 percent fat, and the remained
carbohydrate.  [Canned food generally has more protein and fat but the label
percentages may make it seem lower because of the water content].

LABEL RECOMMENDATIONS
All pet foods come with Recommended Feeding instructions.  The problem is
that these recommendations are NOT absolute requirements even though most
pet caretakers think they have to feed their pet the recommended amounts.
Most house kept dogs and cats, if fed at the amounts stated in the label
recommendations, will eventually become overweight.  Pay attention to your
pet's body weight (size) and just by simple observation decide if it is
overweight.  If so, don't feed so much.

Feeding the "Recommended " daily portions indicated on pet food labels will
nearly always result in feeding more calories than the animal needs for an
average day's energy requirements.

What we should do.
Tailor the amount fed to the cat's body character and physical activity.  If
the cat looks and feels overweight. it is!  You are feeding too much for
that cat's daily needs for energy for exercise or physical activity; and
regardless of what the pet food label's suggested amounts to feed are, you
must feed less than that if the cat is to have a normal (healthy) body
weigh.

http://www.thepetcenter.com/imtop/catweight.html