All dog foods don't have the same calories per cup (better quality foods are more highly concentrated than others), and her activity level might have increased since you adopted her. The dog food you are using should have a guide on the package of cups per day for a dog in a given weight range. With such a tiny dog, a small difference in serving size might make a big difference to her.
Is that want the previous owner used to feed the dog? Things change I say feed her more but just a bit at a time--Even some days dogs are hungrier than others.
I COMPLETELY disagree with the answer given about reading the "guide" on the package of dog food. The simple solution is - ASK YOUR VET. Most dog food packaging encourage overfeeding. Use a high quality food without a ton of useless fillers. They will fill your dog up, but they will be hungry again in no time. Most foods with lamb and rice are generally good for your pup, and they will eat less because the food is a higher quality.