A few years ago there used to be a motto saying Skinny controllers, fat models. It was good at the time, because people thought all of the application code belonged into the controllers, and it helped them realize that it's good to have just a simple controller layer and push things down. The problem with this motto is that it's wrong. Just because you should have skinny controllers it doesn't mean you should have fat models. In fact, none of your classes should be fat.