I thought this was interesting:
Name & Kcal/cup
I'm not endorsing these foods, or suggesting that someone not switch, but perhaps this isn't the best argument to use. The difference is minimal and someone who is convinced to switch from Dog Chow to Chicken Soup because they're told they'll feed less might be sorely dissapointed when they find out Chicken Soup has almost 100 less kcal/cup.
Oh and I know there are a couple of very dense 500+ kcal/cup foods, and I didn't include them because they don't make up the majority and they don't agree with a lot of dogs. Personally I like the less dense foods - easier to digest and the dog gets more bulk and feels more satisfied.