Question about feeding amounts

[*blackrose]

I was recently informed by a pet-drug company sales rep that if you weren't feeding within the recommended guidelines on the back of the bag of dog food (say, for a 50 pound dog 2-3 1/2 cups of food a day or what have you) that your dog wasn't getting the entire amount of nutrition it needed from the food it was consuming.

True, or false? I'm inclined to believe false, but wanted to get other opinions.

I ask, because Chloe only eats a cup of food a day and she is a 45 pound dog. That is what she is maintaining her weight at (she's actually still a bit too heavy for my liking), so that is what she gets. If I fed her the recommended 2 cups of food a day she'd be fat as a blimp. I just wanted to make sure I wasn't depriving her of nutrients because of that.