To reduce the size of Entourage 2008’s database, you can rebuild (or compact) it as follows:
Hold down the Option key and re-launch the application. In a few seconds, you’ll be greeted with the Entourage Database Utility. Select to rebuild [or compact].
Compacting my Entourage 2008 database saved nearly 1.3 GB of space on my MacBook.
Source: Entourage Help Page, Rebuild Your Database (Oct. 17, 2010); see also Microsoft Support, How to Rebuild Your Identity Database in Entourage 2008 for Mac and Entourage for Mac 2004, Article No. 268322 (Sept. 17, 2010).