The higher quality the food the less the dog will need to eat. Anything that is purchased at a big box store is made of lower quality ingredients to keep costs down.
These are some good sites to get a start on finding good foods:
http://www.dogfoodanalysis.com/
http://www.whole-dog-journal.com/
http://www.dogfoodproject.com/index.php
http://www.bornfreeusa.org/facts.php?p=359&more=1
http://www.uncaged.co.uk/petfood.htm
Lower quality ingredients like corn/soy/wheat/byproducts/chemicals/sugars etc are difficult to digest and do not satisfy the dog's hunger.
Higher quality ingredients are digestible and more satisfying so the dog will naturally need to eat less.
My 120lb Newfie gets 2-3 cups/day and my 60lb lab cross gets 1 cup/day; they in perfect shape and are very satisfied.
If the dog is really overweight then sometimes going on a reduced calorie like Natural Balance (which uses good wholesome ingredients) to get to the desired weight faster is an option.
Going to an independent pet store that specializes in natural products would be best.
Hope this is helpful.