My maltese is about 4 lbs and she eats quite a bit. I don't restrict her food. If she's hungry I feed her.
Same thing goes for my 11lb yorkie. If he acts hungry, I feed him more.
Are you giving enough protein or enough calories?
What exactly are you feeding?
Nutrition facts, calories in food, labels, nutritional information and analysis – NutritionData.com
You can use this website to determine the number of calories in each of the foods that you are feeding.
A small dog should typically get 40 Calories per pound of weight. More if they are active, less if they are inactive.