Every dog is so different. Sammy is skinny and 6 lbs and Loki is solid but not fat at 12 lbs, and they eat almost the same amount. 1/4 cup sounds right for Sammy. Loki gets a bit more. You truly have to feel her and look at her to tell if she is over or underweight. Don't base it on if she's hungry, because Loki would weigh 25 pounds if I let HIM decide how much to eat.
You can also switch foods to something better. Try to find a small, independent pet store (none of the chains) and talk to them about good foods. Maybe narrow it down to a few that are available in your area and then I'm sure everyone here would be willing to give you their opinions
Here's some good foods you can start looking for: Merrick, Natura (California Natural, Innova), Nature's Variety, Candiae, Taste of the Wild, Artemis... There are so many more but that will get you started!