Deletion of closed questions is not happening to the extent required. Although the Review Tab for users with enough reputation is available, few of us are actually going in and deleting stuff.
I think the problem was aggravated after we got the review pane with closed & reopen questions. We seem to forget to delete the questions.
Another issue is that recent changes (say, about a year ago?) don't allow closed questions to be deleted for 2 days, because we want to give OP the opportunity to edit and reopen the question. By this time, we forget about the question.
Maybe one solution is to have "Delete" questions in the review list.
- Current list of closed questions
- We had discussed this some time back and agreed not to delete questions marked "Duplicate" as they may bring in traffic. The question is at Deleting Closed Question of No Value is not happening.