How much dry food should I feed my Pixie-bob?

My Pixie-bob is 16lbs. My dad was taking care of her for a couple months and when I took her back she was much heavier than she was when he initially took her. I don't think he portioned how much food she ate, but when she's with me I feed her half a cup a day. She is a large cat and is built like a football player, so I am wondering if I'm not feeding her enough.