Maybe Tim will join us and clarify, but I'll give my 2 cents of understanding.
Questions that were once used can never be deleted, otherwise, you would lose historical information. They are flagged as "deleted" in the MySQL database
. The switch that Dominique mentioned "Also show old questions" I think is then supposed to essentially hide these deleted questions.
It might be that if you delete the quizzes or the students who were once given these questions, that they might disappear in the question bank.
Unless you have a million questions, I doubt that these questions by themselves are making your Moodle backups too large. Sure, every byte that you don't need could help. When I look at the size of my MySQL
tables, yep, it is the quiz attempts tables that are pretty big. But this is due to me allowing students to redo quizzes multiple times (in every one of my courses) and they do. I'll share a screenshot showing some of what I see, tables sorted by size. The logstore table, which keeps track of every click, is typically the largest. How does your's look?