
My sister took in my uncles cat. He was a big boy - VERY VERY OBESE weighing in at 44lbs. This cat was so big, he actually got stuck under the bed, and my sister had to take her bed apart, to get him out.
Anyways, the cat was with her for just over a year. During that time, with the advice and constant supervision from the vets, the cat went on a diet. They started feeding what a 17lb cat should eat. (The vet said it would be his target weight). Slowly, but surely the weight started coming off.
From his obesity, the cat had magnesium crystals. So he was on some meds, and a special veterinary diet for the urinary tract.
Eventually, though, sadly, from a lifetime of being obese, and having health problems associated with it, the cat had to be PTS. He started getting real ill, having seizures and bleeding from the urinary tract.
When he did have to be pts, the vet weighed him before to see what category for euthenasia he was. My sister had gotten this cat down to 25lbs. But unfortunately help came to late to spare him of medical problems associated with a lifetime of being obese.