Spam Is Deleted Automatically After Some Time
If you get lots of spam, the Bulk folder can grow quickly, and faster than Yahoo! Mail will clean it automatically: messages in the Bulk folder are deleted automatically after 30 days.
Fortunately, however big your Bulk folder grows, it does not count toward your Yahoo! Mail storage quota. But even if you let it grow large, it might make sense to clean it once you have inspected the messages in it for good mail. There's no point in seeing the same junk twice.