One of the good and sometimes bad things about Outlook is that it maintains a local copy (cache) of your inbox. Obviously, as your mailbox grows so too does the cache. Where you typically see a problem is when you go to migrate Exchange servers, because generally the cache had to be recreated and all the items downloaded. Thus, if you have a 20GB inbox and you migrated it to Office 365 you’d need to generally get that 20GB to the ‘cloud’ and then allow the 20GB to synchronize to your local cache. Not much fun with very large mailboxes and very poor bandwidth.
As you can see from the above screen shot, Outlook 2013 actually allows you to set how much data will be cached. The minimum you can select is 1 month while the maximum is all (the only option previously available with Outlook). This is really handy in a shared environment like on a Terminal Server where you want to keep the local profile small.
Yet another great reason to upgrade to Office 2013!