The viva isn’t just a big quiz.
Questions could be big. Questions can be about more than what’s in your thesis, but your examiners have more useful things to do than test you on your the general knowledge of your field.
Yes, you need to know about more than what’s in your thesis. No, you don’t have to know everything (remember, you can’t know everything).
(remember, your examiners don’t know everything either)
You know a lot. You needed to, in order to complete your thesis. Most of your examiners’ questions aren’t testing your memory. They want to see how you think. How do you think about your work? How do you think about your field? Knowing things about both is useful, but they know you can’t know everything. What you know will be enough.
The viva is lots of big questions, but not a big quiz.