Not to pull an "ACKTUALLY", but the weight of the cat depends on the cat breed - some cat breeds, such as Maine Coons, can be up to 20 pounds and not be considered fat. But since Peetie is a regular tabby, he should be in that weight range. I'm not Jackson Galaxy, but I'd bet he overeats because...
- the food is avaliable and he's overfed so that way Sapphire's family won't be bothered by him later
- Sapphire can't be bothered to set boundaries with their cat
- to cope with the fact Sapphire is constantly picking at his corns and other wounds
but that's just speculation. It could be a bunch of different things.