If you are trying to generate a class diagram out of a large code base you are going to get a large diagram.
You may want to consider a component, or deployment diagram if you want a high level view of your code.
If you have a code base with 1,000 classes in 100 packages all interrelated (not uncommon), your diagram is going to be hard to read no matter how you cut it. That problem is one of the main reasons for component and deployment diagrams.
In large code bases I generally generate a component diagram for how packages are related, and then a class diagram per package.
If that is still hard to visualize you probably have a complexity issue in your code, and may need to refactor. Cognitive complexity is the number one cause of bugs, and the number one things that slows a development cycle.
If you are having issues diagraming your system in a readable way, developers are going to have a even worse time with it.