Grow wheat grass and try letting it eat it. I had some growing, first my dog got to it and destroyed it. Then this damn cat got to it and now I need to start all over, lol. It was funny cause once my cat started eating it, I walked in and it was protecting it like it was a fucking kitten.
Put it on a diet. Separate the two while feeding so the fat one doesn't cheat and eat the smaller one's food.
Easiest solution in the world: laser pointer. My cat loses her mind for that tiny red dot, she'll chase it anywhere. Flash the laser pointer on the ground, and I'm pretty sure any cat will chase it, therefore getting some much-needed exercise.
Train it in the art of feng shui, it shall then want to live healthier and will naturally begin to lose weight.
If you live in a multi story house. Then just keep bringing your cat to the bottom level. That will force it to constantly walk up the flight of stairs, thus causing your cat to lose weight. Its fool proof... unless all of your cats necessities are downstairs. Then you got yourself a forever fat cat....