My cat, Frankie (black and white at the back in avatar photo) is fat. He is about 5.5kg, we don't know how heavy his sister is as she attacked the vet when we tried to weigh her.
Frankie is a greedy cat and is on steroid tablets every day. This makes him an exceptionally greedy cat. He looks pregnant but doesn't look really fat.
As a fat human, I have always been cautious not to let my dogs or cats become too fat. But I admit to giving them a treat or two to stop the whining - just for a quiet life.