I have 3 cats...( 10,9 and 3)
They get fed twice a day (they sleep the rest!) and only get 1/4 cup of food at a time. i always feel a bit guilty not giving them more but they are still on the chunky side so they must be getting enough.
Have you tried light food?
I also feed my 4 dogs twice a day and depending on their size, they get a cup or less. they too are not thin but on the blump side.
i know with a cat being overwieght, you have to worry about the diabetis thing. i had a diabetic cat for about a year. it really wasn't that bad. so they can live with it!
good luck.
Ducky