Each cat is different. My female, who happens to have a shorter body, is perfect at about 7-8 pounds. My male, who has a longer body, would be perfect at 10-12 pounds if he would ever eat enought (he's below 9 pounds right now). My boyfriend's family has 5 cats, three of which are hugely overweight and are between 16 and 20 pounds. One is underweight at about 5 pounds (she should be 6 pounds, but she's very petite), and the other has lost weight due to illness.
Basically, it depends on the cat.