overweight cats

I have five cats and only feed them one can of cat food  between them all in the  morning with a bowl of dry purina cat food between them all with a can at 5 pm at night no dry food but they are all overweight except the older one who is about 8 ( the others are only three. why is this ?

It depends a lot on what's in the food, not just the amount. Carbs are pretty much useless for a cat nutritionally and often end up being converted to fat, but they are very common in pet foods because they're cheap. Before I got my cat she was being fed only canned by her foster parent and he kept cutting down her portions but she still kept gaining weight, because one of the main ingredients in the food was potato starch. I eventually switched her to a higher protein, very low carb food, and I did have to do some scaling back on her calorie intake before she started losing any weight, but she was MUCH less hungry than she had been on the potato-filled food so it was a lot easier to do so.

Also, cats, like people, have different metabolisms from each other, so what is a normal daily caloric intake for one cat could be much more than what some cats need and much less than others.

You should make sure your cats have been checked out by a vet so that their weight gain isn't from any underlying medical problem. 

Then, you can look at the main ingredients in the food you're feeding. These are the first 5 ingredients for Purina pro plan dry:

Chicken, brewers rice, corn gluten meal, poultry by-product meal (source of glucosamine), dried egg product

Brewers rice and corn meal gluten are filler carbohydrates. The guaranteed analysis lists protein at 40%, but corn and rice make up some of this protein, and cats are obligate carnivores and don't have much use for plant-based proteins.

Having multiple cats makes it difficult to absolutely control how much everyone is eating, but if you can feed everyone separately it will help you keep track of calorie intake and weight loss for each cat and make sure that no one is getting to little. 

Switching to a higher protein food if you can is going to make weight loss a lot easier for everyone. Portion control is also key, but weighing your cats weekly is a good idea because to safely lose weight, a cat should not lose more than 2% of its body weight a week. Exercise, if you can get your cats to partake in playtime, will also help with weight loss.