My Fat Cat

I have a very fat cat. I've tried to put her on a diet but she just sits in front of her bowl and stares at me with her big, round green eyes until I feed her more. What is more, she tries to eat the food of my poor normal-sized cat. I feed them at the same time - the fat cat gobbles her food as fast as possible and then stares at the other cat eating until he is so intimidated that he gives up. Then she pounces on his food and tries to swallow as much as possible before I can take away the bowl. It's driving me crazy.