Exchange Cache Size Change
For years Outlook has had a "cached" mode which keeps a copy of your Exchange data on your local machine as well as on the server. This is a handy feature for many reasons including the ability to work offline, and enabling the instant search which I (frankly) couldn't live without!
In Outlook 2013 the feature has been improved to allow you to choose how much data you wish to keep on your local machine. In the past there was no choice and default was "everything" however Outlook 2013 defaults to the last 12 months' worth.
If you want to change this setting, go to File > Account Settings as illustrated below:
Then select your email account from the list, and press "Change"
Then choose the Cached Exchange Mode Setting that suits your preference: