Tracked this post http://www.olomon.com/wp/2012/06/27/ssrs-solution-to-size-necessary-to-buffer-the-xml-content-exceeded-the-buffer-quota-error/ down when looking into what looks to be a newish error in SQL Server 2012 Reporting Services. You select Preview on the report and get the error “size necessary to buffer the XML content exceeded the buffer quota”.
The blog discuss a good workaround approach. In my case, fields referenced within an expression on the report i.e. a texbox, were referencing fields no longer available in the dataset.