my server can't add maildirsize file under a mail account which has a quota

I installed postfix, virtual user , mysql database, postfixadmin and dovecot.
it works. But I have a problem. when I add a mail account with quota, I can't see maildirsize file in the mail account's maildir directory. However quota feature works.
Whats wrong with it?
How can I correct this ?

cat dovecot.conf

auth_debug = yes
mail_drop_priv_before_exec = yes
protocols = imap imaps pop3 pop3s
disable_plaintext_auth = no
ssl = yes
ssl_cert_file = /etc/ssl/dovecot/cert.pem
ssl_key_file = /etc/ssl/dovecot/key.pem
login_greeting = ISP Mail Server Ready.
mail_location = maildir:/usr/local/virtual/%d/%n
first_valid_uid = 125
last_valid_uid = 125
first_valid_gid = 125
last_valid_gid = 125
protocol imap {
mail_plugins = quota imap_quota
imap_client_workarounds = outlook-idle delay-newmail
}
protocol pop3 {
pop3_no_flag_updates = no
pop3_uidl_format = %v-%u
mail_plugins = quota
pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
}
auth default {
mechanisms = plain login apop
passdb sql {
args = /usr/local/etc/dovecot-sql.conf
}
userdb sql {
args = /usr/local/etc/dovecot-sql.conf
}
userdb prefetch {
}
socket listen {
client {
path = /var/spool/postfix/private/auth
mode = 0660
user = postfix
group = postfix
}
}
}


Thanks