Obese Pets: Mapping out, the weighty matter of our overfed dogs

My bf's mom's cat is huge, which she likes. She wants the cat to be really fat, even though I've told her that it can lead to health problems. I've given up trying to advise her on stuff like healthier cat food because she just says that it's all the same.
 
I've always fed my two dogs their own food mixed with mine--in moderate amounts. They were never fat, always lean, active, and healthy. It's hard not to feed them more when I look at their eyes, but someone has to control their intake for their own sake.