I would agree with silleb. Even if the food has the same ingredients in it, there are going to be different ratios of the said ingredients. Thus, needing different amounts of the food would be needed to achieve optimal health. Also, the amount needs to be adjusted depending on your dogs age, activity level, etc.
It has to do with the ingredients. Higher grade foods require less volume. They will also make the stools tighter and easier to pickup. We give Bodie 1 1/2 cups twice a day of a high quality food. And we might need to lower it to a cup twice a day if he gains more weight.
As a general rule, the better quality the food, the less you need to feed of it. They don't use as many fillers and contain more nutrients that the dog will actually digest and not just pass through them. People say that they cannot afford a higher quality dog food. However, many times it isn't much more expensive since you don't have to feed as much. Jersey gets 3/4 of a cup of Innova small bites twice a day. Sometimes she'll get a little more if she's been more active that day.
It's based on the ingredients. The more named meats a food has, the less you would need to feed. If the first ingredient is something like chicken meal, it doesn't have as much protein as real chicken. So they add in other fillers, which is why you need to feed them more for them to be full.
Regal is about 23 pounds and eats a half cup of Wellness 2x per day.
In addition to the food quality, the individual dog may require a different amount. My four dogs are fed the same food. 3 Beagles-1 Beagle/Lab. Maya gets 1 cup per day (Lazy Beagle), Tate gets 1 1/4 cups per day (slightly more active), Sulli gets 1 3/4 cups per day (active), Sam gets 2 cups per day (very active). We feed Kirkland's Signature (Costco) brand. I have to be careful of how much Maya gets because she has a tendency to put on too much weight. The others are not as much of a concern. I also find that the time of year makes a difference. In the summer time they are all a little more active so their portions are often slightly increased.