Actually, a good code smell / architecture smell in a Rails app is if any given controller method is over 10-15 lines (And even that is on the large side IMHO).

Actually, a good code smell / architecture smell in a Rails app is if any given controller method is over 10-15 lines (And even that is on the large side IMHO). For example, I believe Code Climate specifically penalizes your score for long methods in controllers. I write plenty of Rails apps that have thin controllers and fat models. Class methods / static methods on models can help encapsulate business logic in any language's MVC frameworks, as well as liberal use of the Tell Don't Ask pattern.