Content Database Size details and Move-SPSite

We have calculated the size of all site collection of in our environment using following cmdlet

Get-SPSite | select url, @{label="Size in MB";Expression={$_.usage.storage/1MB}} | Sort-Object -Descending -Property "Size in MB" | ConvertTo-Html -title "Site Collections sort by size" | Set-Content sc.html

When we sum the size of all the site collection it turn out to be 100 GB but when we look at the size of content database, it turns out to be 130 GB. So, do we have any issue in the cmdlet or we are missing something.

Second question is that we will create another content database and shift some of site collections in the new content database using "Move-SPSite" cmdlet. This means that the size of the database which previous hold site collection will automatically reduce or we need to perform any other step.

If we don't need to do anything and wait, then, after how much time the size of the content database will be reduced. 

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