The Single Responsibility Principle tells us that classes should have a single responsibility. If that’s the case, it should be easy to write it down in a single sentence. Try it with one of the big classes in your system. As you think of what the clients need and expect from the class, add clauses to the sentence. The class does this, and this, and this, and that. Is there any one thing that seems more important than anything else? If there is, you might have found the key responsibility of the class. The other responsibilities should probably be factored out into other classes.
