Are you using 6oz cans? If so and you're making one last 5 meals, that is a pretty tiny amount of food.
What brand of food are you giving her?
Switching to something very low carb should be able to give you the weight loss results without having to be quite so restrictive on the amount. Plus as was mentioned, higher protein food will help her feel full longer.
Our female cat was pushing 13lbs after steroid therapy for a skin condition. Since switching to a quality almost all canned diet about 18 months ago, she's now under 11 and continuing to loose.
I feed Innova Cat and Kitten food (this one is not quite as low carb, but they really like it so I rotate it in), Evo Cat and Kitten, Evo 95% Chicken and Turkey and sometimes Wellness Chicken or Turkey. On the rare occasion they get dry food, I use Innova.
HTH!
Edited 10/26/2009 9:53 am ET by sirrahn