Why does mailbox size increase by 30-40% when moved to Exchange 2013/2016?

We get this question very often and its answer is right on the Exchange’s release notes “Mailbox size increase when migrating from previous Exchange versions”.  However, I wanted to provide a little more detail along with good resources already on the web. In Exchange 2013/2016 the mailbox size calculation, includes space that was not accounted for before, but effectively was part of messaging…

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