how to reclaim disk space mdbox

When I delete messages in the archive namespace they are copied to the deleted items folder in the inbox namespace. This is 34MB increase is visible however I expected the archive to go down by 34MB.

Is diskspace reclaimed automatically or do you need to issue an command?

[@ sbin]# du -s -k /home/popusers/testtest /home/mail-archive/testtest 135472 /home/popusers/testtest 358824 /home/mail-archive/testtest

[@ sbin]# du -s -k /home/popusers/testtest /home/mail-archive/testtest 169368 /home/popusers/testtest 358824 /home/mail-archive/testtest

PS. I have mdbox_rotate_size = 4 M