How do you put a cat on a diet?

Buffy is a tad plump. We have a lot of play time (fishing wand toy, I
toss golf balls and glitter balls and she chases them). She also runs
around playing all by herself with the golf balls.

She generally gets a half a can of Sheba pate a day. I dole out 1/4 cup
dry kibble (Purina One Indoor), once in the morning and again at bedtime
so she'll have something to snack on during the night. It doesn't seem
like a lot of food. Yet she weighs 13 lbs!

Is she simply predisposed to be a rather rotund little girl? She's
about 7 years old, maybe 8. I don't really know.

I have gradually cut back, 1/3rd of a can of wet food and slightly less
of the dry. I have discovered that she's sort of rationing herself when
it comes to the dry food. Used to be the dry food was completely gone
by morning. Now there are always a few crunchies left when I get up.

Buffy is perfectly healthy (wish I could have said that about Persia;
she cost me a fortune in prescription food and vet bills over the years!).

Any suggestions? Or do I just have a cute, charming, chubby cat?

Jill