According to Maildir specifications, every user has a cur toplevel directory (not file) . Actually this is his INBOX and then there are .Sent/cur .Trash/cur .Drafts/cur and so on
So if you have a cur directory that is 59G it must belong to a specific user, so the absolute path to it should tell you who is the user.
There is a python script
that you can use to delete mails from IMAP folders.