"Content indexing creates an index, or catalog, that allows users to easily and quickly search through their mail items rather than manually search through the mailbox. Exchange 2007 creates an index that is about 5 percent of the total database size, which is placed on the same LUN as the database. An additional 5 percent capacity needs to be factored into the database LUN size for content indexing."
Extracted from:
http://technet.microsoft.com/en-us/library/bb738147%28EXCHG.80%29.aspx
