Multi-dbox storage space

[MRob]

Hi, I originally picked mdbox because I had the impression from reading 
about it on the mailing list that it was more performant and that it 
would conserve disc space.

But lately i found mailboxes have nearly double the storage as reported. 
I mean, IMAP QUOTA reports around 900MB for one account that has over 
2GB when I inspect the mail location of the user using the linux command 
`du`

Another account has around 750MB according to IMAP but using du I find 
1.5GB. This is for message files and index files combined.

Is this normal? I tried to read the MailboxFormat/dbox wiki page and it 
does not directly address this topic but it does reference "doveadm 
purge". Do I need to use "doveadm purge"?