You are right to be concerned. dogs get the same diseases that humans do - so obesity leads to heart problems and diabetes.
Quantity of food is important, so cut down the food as suggested by the answers you already have. As long as your dog is still acting normally and has the energy to exercise, then there is enogh food being provided.
Make sure you feed good protein, and some vegetables. keep it balanced so there is more protein.
Keep away from any of our processed food for humans(breads etc), they are just as bad for dogs as they are for us.
Fresh meat is much better than processed dog food (in general this is true, there are a few good ones and you have to read the labels to avoid chemicals and by-products).
I have found that giving the dogs a multivitamin does wonders, for health and also for weight control. There are some good dog nutritionals on the market. Flax seed oil is very good for dogs too, so put a few teaspoonsful of this into the meal.
And just like humans, weight loss should be gradual to be healthy.
good luck
Janet
Melbourne, Australia