Migrated Mailboxes Still in Source Database?

I'm running Exchange 2013, hosted on-site.

I've started spreading out my mailboxes into multiple databases.  Yesterday I migrated a number of mailboxes to a new database.  When I look at space consumption (using Get-MailboxStatistics), I see the mailboxes that I moved are still listed in the source database as well as in the destination database.  What do I have to do to recover that space in the source database?

Thanks