I checked the size of my spam folder for August today, and noticed that it was over 1GB in size! I deleted the 12600 "spam" mails received so far this month. There's no way I can go on saving old spams anymore.
Two hours later, the mailbox is again 39MB consisting of 400 mails (each sobig.f mail is about 100K).
Thanks to bogofilter and a simple sobig.f-filter in my procmailrc, they don't interfer too much with my regular emailing.