The previous owner was prolly one of those little elderly people who feeds their pet cookies all the time. My grampa couldn't keep his poodle's weight down because his mom kept feeding them cookies.
I realize it's the camera angle, but Garfield's head looks bigger than the woman's who is holding him. He doesn't look like a domestic-sized cat in this picture.
He facial structure is similar to a Main Coon, if he is one, he was already big to begin with. Now he's just a big fat cat. haha I guess that is why they suspect he'll only loose 15lbs or so.