I took in a pair of sibling cats after their owner died. The owner had been sick for a long time and hadnt been able to tend them properly. The male was so obese he could barely raise or turn his head. It was terrible to see. Not a large-framed cat, he weighed well over 22 pounds. (I couldnt bear to weigh him until I made some progress with him, so cant say for sure.).
Luckily, he was still young enough, 5 years old, that he wanted to play. So I tied a twist tie onto a long string and enticed him to chase it, and then we ran around the house, over and over. His sister took her turns, too. I did this a few times a day for as long as I could keep him interested. It worked pretty quickly. I did not really restrict his food, but did, with the vets advice, put him on a dry formula for overweight cats and measured it.
I know it was the exercise that made the difference. He never was slender, but he became in a few months a more normal looking creature, quite lively and handsome. At his prime he weighed about 13 pounds, I think.