Neutered cats (both male and female) are more likely to develop obesity than their entire counterparts. The precise mechanisms are not fully understood - but there are several likely reasons why this occurs : Normal concentrations of circulating sex hormones reduce appetite but following neutering sex hormone concentrations fall and appetite increases. Reduced male or female sex hormone concentrations in the blood causes a decrease in the body's metabolic rate - i.e. the rate of tissue activity, and the rate at which metabolic processes occur in the body. The metabolic rate in neutered animals has been found to be up to 25% less than in intact animals - which means that they need up to 25% less energy intake. 

So, the reason why neutered animals gain weight
is a combination of : decreased resting energy requirement relative excess food intake, and reduced exercise

Obesity can be usually be prevented by : restricting energy (food) intake to maintain optimum body weight and by specially formulated obesity-control diets. High protein, low carbohydrate diets seem to be a major factor in controlling weight. Also by avoiding high energy snacks and treats and  maintain a regular exercise regimen

 
Weight control in your cat is dependent both on the amount of calories your cat eats and the percent protein in the food.  
 


High carbohydrate food (dry foods with only 35% protein or wet food with only 8% protein) cause your cat to produce more insulin and drive the carbohydrates into the fat pads even if the total caloric intake is low. Not all cats have this problem but if your cat is fat you need to use a food high in protein and low in calories to lower its body weight. The table below lists the amount of high protein food that is needed to achieve a desired weight.
 
Failure to control the weight of your cat will predispose it to diabetes.