If a committee asks for major revisions after defense, you must complete them to get the signatures needed for your degree. But this shouldn't happen. The chair should catch major problems before defense and only let you defend when minor revisions are all that's left.
What happens if the committee asks for major revisions after the defense?
So, when you defend your dissertation, your goal is signatures. And that means they sign the title page— whoever has to sign for your university. So, if they've asked for major revisions, then you have to do them to get the signature. However, you probably shouldn't have gotten to that point. The major revision should come before the defense because your chair shouldn't let you defend unless they know that you're going to be moving forward with, at worst, minor revisions.
