Get user mailbox script

Hello, can someone please tell me why this particular one doesn't return any values for the mailbox size?  It works if I add the mailbox size property to the objects later, but I would think this should work too no?

$test=get-mailbox -resultsize unlimited | select ServerName,OrganizationalUnit,recipienttype,PrimarySMTPAddress,DisplayName,Database,@{
        name="Mailbox Size (MB)";expression={(get-mailboxstatistics $_.displayname).totalitemsize.value.tomb()}}