My daughter found a huge Maine Coon who had originally had an owner, but had been homeless for some time. My DD didn't understand that he shouldn't be allowed to eat all he wanted besides the fact that he was a hunter & moocher in the apt. complex where she lived. He got up to 28 lbs!
I ended up with him & the vet said that even though he was so long & big, with a huge head, he should ideally weigh between 17-18 lbs. I put him on senior food & monitored him, but the least I ever got him down to was 18. Then he rapidly went back up to 20 & stayed there. He is gone now, but was the best cat I ever had. His personality was much more like a dog than a cat & he was friendly with everyone. Unfortunately, he got really bad arthritis & was in so much pain I finally had to put him down, but he was old by then. We didn't know his age, but the vet estimated at least 18 yrs.

I now have a regular housecat (Phyllis) that I've had since Halloween & she's gone from perfect 9.9 lbs to 11.8 since I've had her! She was previously an outdoor kitty with her old owner & I know it's the lack of exercise. I'm going to have to stop the free feeding and try to get her moving more....