Fat people have a bigger fat layer around their muscles, but they do not always stay warmer. Many "skinny" people are thin because they are quite fit and have a lot of muscle; muscles make you warmer. If the thin person is thin because they are just undernourished and in poor shape, then the fatter person will be warmer.
You probably shouldn't be feeding your cat milk daily. Only sometimes. And if you feed it the right ammount of cat food, it will not get fat. We excersise our five cats ten to fifteen minutes daily. They play with laser pointers and love to chase toy mice. If you take proper care of your cat, it will not get fat.
well, there was a cat, it was fat, it wore a hat, and after that, it layed on a mat, then chased a rat, the rat was fat, it wore a hat, layed on a mat, got chased by a cat, the cat was fat, it wore a hat, after that, it layed on a mat, then chased a rat, the rat was fat, it wore a hat, after that it DIED.