With the release of SharePoint 2013 Service Pack 1, you can now re-calculate site storage metrics on demand. This is done via PowerShell.
$site = Get-SPSite http://siteUrl
This method executes proc_RecalculateStorageMetricsForSite, which can be found in each Content Database. The Content Database must be at the SP1 schema level.
Storage metrics can be found on each site by visiting http://siteUrl/_layouts/15/storman.aspx.