I would think the best way to see what is going on is to do some tracing, the query in SQL Server, the network traffic using Wireshark or similar, and the HTTP stream using fiddler or similar. The SQL trace will show you if there are any strange results being returned by the query versus the manual execution, the network trace will show you the amount of traffic and any issues with TDS from SQL to the report service, the XML to the web server, and the web server rendering to the client. The HTTP trace will show any large objects from the HTTP pipeline and may help pinpoint which subsystem is the cause of the issue. Keep in mind that the error is pointing to the HTTP request length, so I would actually start by looking at the request from the client to the web server, and then check the request from the web server to SSRS first. Perhaps something in the report RDL is causing a rather large HTTP request to be sent.