Skinny chihuahua

I have a chihuahua, ever since I got her from someone she has been skinny. I feed her kibbles and bits (which is her favorite) for small dogs twice a day, sometimes three but people say you're only supposed to feed a dog twice. I give her a 1 1/2 handfuls each time. If I give her more she pukes. Shes more than a year. She's always been the same size but people say she's too skinny and it's annoying. Am I under feeding her or something?