Glad to hear you've resolved the issue ... about your PM concerning submitting to Tracker. May as well. And thanks for providing exactly what you did ... there's a shared query for custom_sql addon that shows largest tables, BTW, and have had to use it in troubleshooting issues on sites before.
Guess there is a reason that quiz attempts logs were kept. The quiz.xml file is asciii and 323 MB does sound like a lot of questions.
If the old site still up, there is a hidden admin tool to check the health of Moodle.
http://site/admin/tool/health/
If there are issues with quizzes the health check does report them as well as possible fixes via SQL commands/statements one could run via CLI or any tool that could execute SQL on the DB for the Moodle.
Wonder what that might report.
'spirit of sharing', Ken