There is One Model One Table pattern. One place to look for anything related to corresponding database table.
There is Single Responsibility Principle. Splitting model into several standalone actions.
There is Model As Layer philosophy. Meaning that model is made of three types of structures: Domain Objects (completely unaware of the storage), Data Mappers (only responsible for the storage) and Services (that interact between Domain Objects and Data Mappers).
What do YOU use?
Having so many approaches (and I suppose there are more) I’m having problem even understanding how should I organize Controllers and Models to go with the most convenient solution.